Remove internal header inclusion guards

Inclusion of internal headers is already both centralized and follows
strict rules.  Inclusion guards are both an unneccessary maintenance
burden and may hide inclusion order bugs.

src/posix_time.h

@@ -25,9 +25,6 @@
 //
 //========================================================================
 
-#ifndef _glfw3_posix_time_h_
-#define _glfw3_posix_time_h_
-
 #define _GLFW_PLATFORM_LIBRARY_TIMER_STATE _GLFWtimerPOSIX posix
 
 #include <stdint.h>
@@ -45,4 +42,3 @@ typedef struct _GLFWtimerPOSIX
 
 void _glfwInitTimerPOSIX(void);
 
-#endif // _glfw3_posix_time_h_