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

libi3/string.c

@@ -11,11 +11,10 @@
  */
 #include "libi3.h"
 
+#include <glib.h>
 #include <stdlib.h>
 #include <string.h>
 
-#include <glib.h>
-
 struct _i3String {
     char *utf8;
     xcb_char2b_t *ucs2;