Fix POSIX conformance issues for clock_gettime

CLOCK_MONOTONIC should not be used as a feature macro.  The POSIX
feature macros are provided by unistd.h.  CLOCK_MONOTONIC is provided by
time.h.  CLOCK_MONOTONIC requires _POSIX_C_SOURCE >= 199309L on some
systems.

src/x11_init.c

@@ -36,6 +36,7 @@
 #include <limits.h>
 #include <stdio.h>
 #include <locale.h>
+#include <unistd.h>
 
 
 // Translate an X11 key code to a GLFW key code.