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

src/sighandler.c

@@ -12,9 +12,8 @@
 #include <signal.h>
 #include <sys/wait.h>
 
-#include <xcb/xcb_event.h>
-
 #include <X11/keysym.h>
+#include <xcb/xcb_event.h>
 
 typedef struct dialog_t {
     xcb_window_t id;