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/ipc_send_message.c

@@ -7,14 +7,13 @@
  */
 #include "libi3.h"
 
-#include <string.h>
-#include <stdlib.h>
-#include <unistd.h>
-#include <stdint.h>
 #include <err.h>
 #include <errno.h>
-
 #include <i3/ipc.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
 
 /*
  * Formats a message (payload) of the given size and type and sends it to i3 via