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, 1 insertions, 1 deletions

diff --git a/options.c b/options.c
index 8fade87..69b45b9 100644
--- a/options.c
+++ b/options.c
@@ -20,8 +20,8 @@
 #include "nsxiv.h"
 #include "version.h"
 
-#include <stdlib.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>