BUILD: http: remove a warning on strndup

The latest commit about set-map/add-acl/... causes this warning for
me :

src/proto_http.c: In function 'parse_http_req_cond':
src/proto_http.c:8863: warning: implicit declaration of function 'strndup'
src/proto_http.c:8863: warning: incompatible implicit declaration of built-in function 'strndup'
src/proto_http.c:8890: warning: incompatible implicit declaration of built-in function 'strndup'
src/proto_http.c:8917: warning: incompatible implicit declaration of built-in function 'strndup'
src/proto_http.c:8944: warning: incompatible implicit declaration of built-in function 'strndup'

Use my_strndup() instead of strndup() which is not portable. No backport
needed.
diff --git a/src/proto_http.c b/src/proto_http.c
index 8467dbc..90b73da 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -8860,7 +8860,7 @@
 		 * '+ 8' for 'add-acl('
 		 * '- 9' for 'add-acl(' + trailing ')'
 		 */
-		rule->arg.map.ref = strndup(args[0] + 8, strlen(args[0]) - 9);
+		rule->arg.map.ref = my_strndup(args[0] + 8, strlen(args[0]) - 9);
 
 		cur_arg = 1;
 
@@ -8887,7 +8887,7 @@
 		 * '+ 8' for 'del-acl('
 		 * '- 9' for 'del-acl(' + trailing ')'
 		 */
-		rule->arg.map.ref = strndup(args[0] + 8, strlen(args[0]) - 9);
+		rule->arg.map.ref = my_strndup(args[0] + 8, strlen(args[0]) - 9);
 
 		cur_arg = 1;
 
@@ -8914,7 +8914,7 @@
 		 * '+ 8' for 'del-map('
 		 * '- 9' for 'del-map(' + trailing ')'
 		 */
-		rule->arg.map.ref = strndup(args[0] + 8, strlen(args[0]) - 9);
+		rule->arg.map.ref = my_strndup(args[0] + 8, strlen(args[0]) - 9);
 
 		cur_arg = 1;
 
@@ -8941,7 +8941,7 @@
 		 * '+ 8' for 'set-map('
 		 * '- 9' for 'set-map(' + trailing ')'
 		 */
-		rule->arg.map.ref = strndup(args[0] + 8, strlen(args[0]) - 9);
+		rule->arg.map.ref = my_strndup(args[0] + 8, strlen(args[0]) - 9);
 
 		cur_arg = 1;
 
@@ -9160,7 +9160,7 @@
 		 * '+ 8' for 'add-acl('
 		 * '- 9' for 'add-acl(' + trailing ')'
 		 */
-		rule->arg.map.ref = strndup(args[0] + 8, strlen(args[0]) - 9);
+		rule->arg.map.ref = my_strndup(args[0] + 8, strlen(args[0]) - 9);
 
 		cur_arg = 1;
 
@@ -9188,7 +9188,7 @@
 		 * '+ 8' for 'del-acl('
 		 * '- 9' for 'del-acl(' + trailing ')'
 		 */
-		rule->arg.map.ref = strndup(args[0] + 8, strlen(args[0]) - 9);
+		rule->arg.map.ref = my_strndup(args[0] + 8, strlen(args[0]) - 9);
 
 		cur_arg = 1;
 
@@ -9215,7 +9215,7 @@
 		 * '+ 8' for 'del-map('
 		 * '- 9' for 'del-map(' + trailing ')'
 		 */
-		rule->arg.map.ref = strndup(args[0] + 8, strlen(args[0]) - 9);
+		rule->arg.map.ref = my_strndup(args[0] + 8, strlen(args[0]) - 9);
 
 		cur_arg = 1;
 
@@ -9242,7 +9242,7 @@
 		 * '+ 8' for 'set-map('
 		 * '- 9' for 'set-map(' + trailing ')'
 		 */
-		rule->arg.map.ref = strndup(args[0] + 8, strlen(args[0]) - 9);
+		rule->arg.map.ref = my_strndup(args[0] + 8, strlen(args[0]) - 9);
 
 		cur_arg = 1;