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/freedesktop_portal_desktop.cpp

@@ -35,6 +35,7 @@
 #include "core/error/error_macros.h"
 #include "core/os/os.h"
 #include "core/string/ustring.h"
+#include "core/variant/variant.h"
 
 #ifdef SOWRAP_ENABLED
 #include "dbus-so_wrap.h"
@@ -42,8 +43,6 @@
 #include <dbus/dbus.h>
 #endif
 
-#include "core/variant/variant.h"
-
 #define BUS_OBJECT_NAME "org.freedesktop.portal.Desktop"
 #define BUS_OBJECT_PATH "/org/freedesktop/portal/desktop"