[dev] dwm 5.9 small patch for non xinerama users

Hi.

I noted that if i disable Xinerama, i don't use also the following
functions: dirtomon, focusmon, tagmon.
So, i think it is possible to hold all these functions between "#ifdef
XINERAMA ... #endif".
The difference in the final binary file size is very little but...
A possible patch is attached...

Bye,
 Mauro.