fix: use after free, ISO C warning

tests/fmemopen.c

@@ -1,6 +1,8 @@
+#include "fmemopen.h"
+
+typedef int make_iso_compilers_happy; //...
 #ifdef _WIN32
 #include <windows.h>
-#include "fmemopen.h"
 #include <share.h>
 #include <io.h>
 #include <fcntl.h>

tests/fmemopen.h

@@ -1,5 +1,6 @@
-#ifdef _WIN32
 #pragma once
+
+#ifdef _WIN32
 #include <stdio.h>
 FILE *fmemopen(void *buf, size_t len, const char *type);
 #endif