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/window.h

@@ -7,8 +7,8 @@
  * window.c: Updates window attributes (X11 hints/properties).
  *
  */
-#ifndef _WINDOW_H
-#define _WINDOW_H
+#ifndef I3_WINDOW_H
+#define I3_WINDOW_H
 
 /**
  * Updates the WM_CLASS (consisting of the class and instance) for the