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/x11_platform.h

@@ -25,9 +25,6 @@
 //
 //========================================================================
 
-#ifndef _glfw3_x11_platform_h_
-#define _glfw3_x11_platform_h_
-
 #include <unistd.h>
 #include <signal.h>
 #include <stdint.h>
@@ -335,4 +332,3 @@ void _glfwInputErrorX11(int error, const char* message);
 
 void _glfwPushSelectionToManagerX11(void);
 
-#endif // _glfw3_x11_platform_h_