BUILD: sample: use __fallthrough in smp_is_rw() and smp_dup()

This avoids three build warnings when preprocessing happens before compiling
with gcc >= 7.
diff --git a/include/haproxy/sample.h b/include/haproxy/sample.h
index 9ec4df1..14d1a06 100644
--- a/include/haproxy/sample.h
+++ b/include/haproxy/sample.h
@@ -99,7 +99,7 @@
 	case SMP_T_METH:
 		if (smp->data.u.meth.meth != HTTP_METH_OTHER)
 			return 1;
-		/* Fall through */
+		__fallthrough;
 
 	case SMP_T_STR:
 		if (!smp->data.u.str.size || smp->data.u.str.data >= smp->data.u.str.size)
@@ -149,7 +149,7 @@
 	case SMP_T_METH:
 		if (smp->data.u.meth.meth != HTTP_METH_OTHER)
 			return 1;
-		/* Fall through */
+		__fallthrough;
 
 	case SMP_T_STR:
 		if (!smp->data.u.str.size ||
diff --git a/src/sample.c b/src/sample.c
index cd9fccf..a3acfef 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -823,7 +823,7 @@
 	case SMP_T_METH:
 		if (smp->data.u.meth.meth != HTTP_METH_OTHER)
 			break;
-		/* Fall through */
+		__fallthrough;
 
 	case SMP_T_STR:
 		trash = get_trash_chunk();