src: include primary header first

This is a common practice in C projects, which simply enforces that
each header must compile cleanly without implicit dependencies on
other headers (see also the previous commit).

src/common/fd-util.c

@@ -1,11 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0-only
 #define _POSIX_C_SOURCE 200809L
 
+#include "common/fd-util.h"
 #include <sys/resource.h>
 #include <wlr/util/log.h>
 
-#include "common/fd-util.h"
-
 static struct rlimit original_nofile_rlimit = {0};
 
 void