Conditionalize #pragma warn for MSVC.

This warning disable was conditional on _WIN32, which mingw
also defines, resulting in a warning about the unknown pragma
on gcc. Instead make it depend on something only the Visual
Studio compiler defines.
diff --git a/silk/debug.h b/silk/debug.h
index 7e6a357..a71f1e7 100644
--- a/silk/debug.h
+++ b/silk/debug.h
@@ -69,7 +69,9 @@
 
 #if (defined(_WIN32) || defined(_WINCE))
 #include <windows.h>    /* timer */
+#if defined(_MSC_VER)
 #pragma warning( disable : 4996 )       /* stop bitching about strcpy in TIC()*/
+#endif
 #else   /* Linux or Mac*/
 #include <sys/time.h>
 #endif