config: Make sure HAVE_TRUNC and HAVE_TRUNCF are defined as appropriate.

The configure/cmake scripts were checking for these functions but we didn't
have the SDL_config.h.* pieces in place. The other config headers are best
guesses.

include/SDL_config_iphoneos.h

@@ -122,6 +122,8 @@
 #define HAVE_SQRTF  1
 #define HAVE_TAN    1
 #define HAVE_TANF   1
+#define HAVE_TRUNC  1
+#define HAVE_TRUNCF 1
 #define HAVE_SIGACTION  1
 #define HAVE_SETJMP 1
 #define HAVE_NANOSLEEP  1