Fixed snprintf override which breaks on old MacOS

`snprintf` is available in Mac/FreeBSD headers only if _XOPEN_SOURCE is 600 or higher.
This closes #586.

Author: mvp

uhubctl.c

@@ -9,7 +9,7 @@
  *
  */
 
-#define _XOPEN_SOURCE 500
+#define _XOPEN_SOURCE 600
 
 #include <stdio.h>
 #include <stdlib.h>
@@ -35,10 +35,6 @@
 #include <libusb-1.0/libusb.h>
 #endif
 
-#if defined(__APPLE__) || defined(__FreeBSD__) /* snprintf is not available in pure C mode */
-int snprintf(char * __restrict __str, size_t __size, const char * __restrict __format, ...) __printflike(3, 4);
-#endif
-
 #if !defined(LIBUSB_API_VERSION) || (LIBUSB_API_VERSION <= 0x01000103)
 #define LIBUSB_DT_SUPERSPEED_HUB 0x2a
 #endif