Style: Harmonize header includes in platform ports

This applies our existing style guide, and adds a new rule to that style
guide for modular components such as platform ports and modules:

Includes from the platform port or module should be included with relative
paths (relative to the root folder of the modular component, e.g.
`platform/linuxbsd/`), in their own section before Godot's "core" includes.

The `api` and `export` subfolders also need to be handled as self-contained
(and thus use relative paths for their "local" includes) as they are all
compiled for each editor platform, without necessarily having the api/export
matching platform folder in the include path.
E.g. the Linux editor build will compile `platform/android/{api,export}/*.cpp`
and those need to use relative includes for it to work.

platform/linuxbsd/x11/gl_manager_x11.cpp

@@ -30,15 +30,14 @@
 
 #include "gl_manager_x11.h"
 
-#ifdef X11_ENABLED
-#if defined(GLES3_ENABLED)
+#if defined(X11_ENABLED) && defined(GLES3_ENABLED)
+
+#include "thirdparty/glad/glad/glx.h"
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 
-#include "thirdparty/glad/glad/glx.h"
-
 #define GLX_CONTEXT_MAJOR_VERSION_ARB 0x2091
 #define GLX_CONTEXT_MINOR_VERSION_ARB 0x2092
 
@@ -403,5 +402,4 @@ GLManager_X11::~GLManager_X11() {
 	release_current();
 }
 
-#endif
-#endif
+#endif // X11_ENABLED && GLES3_ENABLED