MINOR: http: export the smp_fetch_cookie function

Remove the static attribute of smp_fetch_cookie, and declare the
function in proto/proto_http.h for future use.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 065a0ea..6370e2d 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -119,6 +119,8 @@
 struct chunk *http_error_message(struct session *s, int msgnum);
 struct redirect_rule *http_parse_redirect_rule(const char *file, int linenum, struct proxy *curproxy,
                                                const char **args, char **errmsg, int use_fmt);
+int smp_fetch_cookie(struct proxy *px, struct session *l4, void *l7, unsigned int opt,
+                 const struct arg *args, struct sample *smp, const char *kw);
 
 enum http_meth_t find_http_meth(const char *str, const int len);