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/dpi.c

@@ -9,6 +9,7 @@
 
 #include <math.h>
 #include <stdlib.h>
+
 #include <xcb/xcb_xrm.h>
 
 static long dpi;