Check for _WIN32 instead of GIT_WIN32 or WIN32 to detect windows build environments

This fixes a possible compilation issue (when GIT_WIN32 was not set) which was introduced in revision 69a4bc19.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

src/xdiff/xinclude.h

@@ -29,7 +29,7 @@
 #include <string.h>
 #include <limits.h>
 
-#ifdef GIT_WIN32
+#ifdef _WIN32
 #else
 #include <unistd.h>
 #endif