SDL_config_os2.h: undefine HAVE_TRUNC

trunc() exists only in OpenWatcom 2.0 fork, and its implementation
already is the same as the fallback we have here..

include/SDL_config_os2.h

@@ -180,7 +180,7 @@
 /* #undef HAVE_SQRTF */
 #define HAVE_TAN 1
 /* #undef HAVE_TANF */
-#define HAVE_TRUNC 1
+/* #undef HAVE_TRUNC */
 /* #undef HAVE_TRUNCF */
 
 #endif /* SDL_config_os2_h_ */