Fix a harmless compiler warning

GCC 4.2.4 on gentoo:
 awesome-3.3-rc3/mousegrabber.c: In function ‘luaA_mousegrabber_run’:
 awesome-3.3-rc3/mousegrabber.c:37: warning: ‘root’ may be used uninitialized in this function
 awesome-3.3-rc3/mousegrabber.c:37: note: ‘root’ was declared here

Thanks to gogonkt for reporting this.

Signed-off-by: Uli Schlachter <psychon@znc.in>

mousegrabber.c

@@ -34,7 +34,7 @@
 static bool
 mousegrabber_grab(xcb_cursor_t cursor)
 {
-    xcb_window_t root;
+    xcb_window_t root = XCB_NONE;
 
     for(int screen = 0;
         screen < xcb_setup_roots_length(xcb_get_setup(globalconf.connection));