update size hints on zoom

On font zooming (i.e. xzoom()), window size hints are not updated.  This
patch does that.

Signed-off-by: Roberto E. Vargas Caballero <k0ga@shike2.com>

st.c

@@ -2993,6 +2993,7 @@ xzoom(const Arg *arg) {
 	xloadfonts(usedfont, usedfontsize + arg->i);
 	cresize(0, 0);
 	redraw(0);
+	xhints();
 }
 
 void