libobs/util: Define PRAGMA_WARN_DEPRECATION for MSVC

Define PRAGMA_WARN_DEPRECATION for MSVC to enable using that define for
cross-platform code, which allows us to write slightly less code when we
need to suppress this warning.

libobs/util/c99defs.h

@@ -54,7 +54,7 @@
 #ifdef _MSC_VER
 #define PRAGMA_WARN_PUSH __pragma(warning(push))
 #define PRAGMA_WARN_POP __pragma(warning(pop))
-#define PRAGMA_WARN_DEPRECATION
+#define PRAGMA_WARN_DEPRECATION __pragma(warning(disable : 4996))
 #elif defined(__clang__)
 #define PRAGMA_WARN_PUSH _Pragma("clang diagnostic push")
 #define PRAGMA_WARN_POP _Pragma("clang diagnostic pop")