sort and group includes

* includes are sorted alphabetically

* their grouping and layout is the following:
  - nsxiv.h will be the first include
  - followed by any internal headers (e.g "commands.h" "config.h")
  - followed by system headers (<stdlib.h> etc)
  - followed by third party headers (X.h libwebp etc)

* also add `llvm-include-order` check to clang-tidy so that it can catch
  unsorted includes during CI.

1 files changed, 2 insertions, 2 deletions

diff --git a/util.c b/util.c
index 31ab4b8..65c39e5 100644
--- a/util.c
+++ b/util.c
@@ -20,13 +20,13 @@
 #include "nsxiv.h"
 
 #include <assert.h>
-#include <stdlib.h>
+#include <errno.h>
 #include <stdarg.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <unistd.h>
-#include <errno.h>
 
 const char *progname;