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/godot_linuxbsd.cpp

@@ -28,6 +28,10 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                 */
 /**************************************************************************/
 
+#include "os_linuxbsd.h"
+
+#include "main/main.h"
+
 #include <limits.h>
 #include <locale.h>
 #include <stdlib.h>
@@ -37,9 +41,6 @@
 #include <sys/resource.h>
 #endif
 
-#include "main/main.h"
-#include "os_linuxbsd.h"
-
 int main(int argc, char *argv[]) {
 #if defined(SANITIZERS_ENABLED)
 	// Note: Set stack size to be at least 30 MB (vs 8 MB default) to avoid overflow, address sanitizer can increase stack usage up to 3 times.