sdk: make C++11 plugins build again

compositor.h must not define a 'static_assert' macro, since that
conflicts with the new 'static_assert' in the standard and breaks
the build.

desktop-shell/shell.c

@@ -43,6 +43,10 @@
 #define DEFAULT_NUM_WORKSPACES 1
 #define DEFAULT_WORKSPACE_CHANGE_ANIMATION_LENGTH 200
 
+#ifndef static_assert
+#define static_assert(cond, msg)
+#endif
+
 struct focus_state {
 	struct weston_seat *seat;
 	struct workspace *ws;