ext/curl/config.m4: include string.h for test using strncmp()

Commit 2ecafd4 updates one of the curl ./configure tests to use
strncmp(), but that function may not be defined unless string.h is
included. We add it here.

Closes GH-23208

Author: orlitzky

ext/curl/config.m4

@@ -24,6 +24,7 @@ if test "$PHP_CURL" != "no"; then
 
       AC_RUN_IFELSE([AC_LANG_PROGRAM([
 #include <stdio.h>
+#include <string.h>
 #include <strings.h>
 #include <curl/curl.h>
 ], [