fix foreign test for i3nt

For some build environments, `_WIN32` is needed (as in other
"foreign*.c" files) instead of `WIN32` without the leading underscore.

mats/foreign3.c

@@ -17,7 +17,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#ifndef WIN32
+#ifndef _WIN32
 #include <string.h>
 #endif
 
@@ -178,7 +178,7 @@ EXPORT char Srvtest_char(ptr code, ptr x1) {
     return (*((char (*)(ptr))Sforeign_callable_entry_point(code)))(x1);
 }
 
-#ifdef WIN32
+#ifdef _WIN32
 EXPORT int __stdcall sum_stdcall(int a, int b) {
     return a + b;
 }
@@ -208,7 +208,7 @@ EXPORT com_instance_t *get_com_instance(void) {
   com_instance.data = -31;
   return &com_instance;
 }
-#endif /* WIN32 */
+#endif /* _WIN32 */
 
 /* foreign_callable example adapted from foreign.stex */
 typedef void (*CB)(char);