centeredmaster: fix missing keybinds

Centeredmaster and centeredfloatingmaster options are working fine,
apart from not having the keybinds that allow the user to set their
layouts. This fix addresses this problem by adding the original patch
keybinds  `MODKEY+u` for centeredmaster and `MODKEY+o` for
centeredfloatingmaster.

config.def.h

@@ -1067,6 +1067,12 @@ static const Key keys[] = {
 	#if COLUMNS_LAYOUT
 	{ MODKEY,                       XK_c,          setlayout,              {.v = &layouts[3]} },
 	#endif // COLUMNS_LAYOUT
+	#if CENTEREDMASTER_LAYOUT
+	{ MODKEY,                       XK_u,          setlayout,              {.v = &layouts[3]} },
+        #endif // CENTEREDMASTER_LAYOUT
+	#if CENTEREDFLOATINGMASTER_LAYOUT
+	{ MODKEY,                       XK_o,          setlayout,              {.v = &layouts[4]} },
+        #endif // CENTEREDFLOATINGMASTER_LAYOUT
 	#if FLEXTILE_DELUXE_LAYOUT
 	{ MODKEY|ControlMask,           XK_t,          rotatelayoutaxis,       {.i = +1 } },   /* flextile, 1 = layout axis */
 	{ MODKEY|ControlMask,           XK_Tab,        rotatelayoutaxis,       {.i = +2 } },   /* flextile, 2 = master axis */