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/font.c

@@ -8,14 +8,13 @@
 #include "libi3.h"
 
 #include <assert.h>
+#include <cairo/cairo-xcb.h>
+#include <err.h>
+#include <pango/pangocairo.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
-#include <stdbool.h>
-#include <err.h>
-
-#include <cairo/cairo-xcb.h>
-#include <pango/pangocairo.h>
 
 static const i3Font *savedFont = NULL;