Sort includes in *.c files

Not enabling in .clang-format because it breaks headers files.

Used:
    IncludeCategories:
      - Regex:           '^<config'
        Priority:        0
      - Regex:           '^".*"'
        Priority:        1
      - Regex:           '^<(xcb|xkb|yajl|X11)'
        Priority:        3
      - Regex:           '.*'
        Priority:        2

i3-input/keysym2ucs.c

@@ -31,9 +31,10 @@
  * This software is in the public domain. Share and enjoy!
  */
 
-#include <xcb/xcb.h>
 #include "keysym2ucs.h"
 
+#include <xcb/xcb.h>
+
 struct codepair {
     unsigned short keysym;
     unsigned short ucs;