Add attribute format printf for Q_vsnprintf

Author: timangus

code/qcommon/q_shared.h

@@ -187,7 +187,7 @@ typedef int intptr_t;
 #ifdef _WIN32
   // vsnprintf is ISO/IEC 9899:1999
   // abstracting this to make it portable
-  int Q_vsnprintf(char *str, size_t size, const char *format, va_list ap);
+  int Q_vsnprintf(char *str, size_t size, const char *format, va_list ap) __attribute__((format(printf, 3, 0)));
 #else
   #define Q_vsnprintf vsnprintf
 #endif