don’t use reversed identifiers for include guards (Thanks Markus)

Done with:

    sed -in 's/\(ifndef\|define\) _\([0-9A-Z_]*\)$/\1 I3_\2/' include/**/*.h

fixes #804

include/resize.h

@@ -7,8 +7,8 @@
  * resize.c: Interactive resizing.
  *
  */
-#ifndef _RESIZE_H
-#define _RESIZE_H
+#ifndef I3_RESIZE_H
+#define I3_RESIZE_H
 
 int resize_graphical_handler(Con *first, Con *second, orientation_t orientation, const xcb_button_press_event_t *event);