Misc: Modify function attributes

- Remove the const function attribute from get_time_*(), since the
   functions they call that actually get the time probably access global
   variables or have other behaviors unacceptable for const functions.

 - Change the const function attribute on win_has_frame() to pure, as it
   accesses memory its parameter points to, which is invalid for a const
   function.

src/compton.h

@@ -500,7 +500,7 @@ update_reg_ignore_expire(session_t *ps, const win *w) {
 /**
  * Check whether a window has WM frames.
  */
-static inline bool __attribute__((const))
+static inline bool __attribute__((pure))
 win_has_frame(const win *w) {
   return w->a.border_width
     || w->frame_extents.top || w->frame_extents.left