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/os_linuxbsd.cpp

@@ -35,17 +35,13 @@
 #include "main/main.h"
 #include "servers/display_server.h"
 
-#include "modules/modules_enabled.gen.h" // For regex.
-#ifdef MODULE_REGEX_ENABLED
-#include "modules/regex/regex.h"
-#endif
-
 #ifdef X11_ENABLED
 #include "x11/display_server_x11.h"
 #endif
 
-#ifdef HAVE_MNTENT
-#include <mntent.h>
+#include "modules/modules_enabled.gen.h" // For regex.
+#ifdef MODULE_REGEX_ENABLED
+#include "modules/regex/regex.h"
 #endif
 
 #include <dlfcn.h>
@@ -57,6 +53,10 @@
 #include <sys/utsname.h>
 #include <unistd.h>
 
+#ifdef HAVE_MNTENT
+#include <mntent.h>
+#endif
+
 void OS_LinuxBSD::alert(const String &p_alert, const String &p_title) {
 	const char *message_programs[] = { "zenity", "kdialog", "Xdialog", "xmessage" };