tree-wide: Sort includes

This was done by running a locally built clang-format with
https://github.com/llvm/llvm-project/pull/137617 and
https://github.com/llvm/llvm-project/pull/137840 applied on all .c
and .h files.

src/core/timer.c

@@ -1,11 +1,10 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <errno.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
-#include <errno.h>
-
 #include "alloc-util.h"
 #include "bus-error.h"
 #include "bus-util.h"
@@ -20,8 +19,8 @@
 #include "string-table.h"
 #include "string-util.h"
 #include "timer.h"
-#include "unit-name.h"
 #include "unit.h"
+#include "unit-name.h"
 #include "user-util.h"
 #include "virt.h"