Fixed compile warning

src/file/SDL_rwops.c

@@ -23,8 +23,10 @@
    configure script knows the C runtime has it and enables it. */
 #ifndef __QNXNTO__
 /* Need this so Linux systems define fseek64o, ftell64o and off64_t */
+#ifndef _LARGEFILE64_SOURCE
 #define _LARGEFILE64_SOURCE
 #endif
+#endif
 
 #include "../SDL_internal.h"