libobs/util: Add function deprecation defines

libobs/util/c99defs.h

@@ -24,8 +24,12 @@
 #define UNUSED_PARAMETER(param) (void)param
 
 #ifdef _MSC_VER
+#define DEPRECATED_START __declspec(deprecated)
+#define DEPRECATED_END
 #define FORCE_INLINE __forceinline
 #else
+#define DEPRECATED_START
+#define DEPRECATED_END __attribute__ ((deprecated))
 #define FORCE_INLINE inline __attribute__((always_inline))
 #endif