[dwm] 'make clean' and config.h

Hi,

Currently 'make clean' never removes config.h. In case of a custom
config.h this is of course desired behaviour. But when config.h is
copied from config.default.h it can be safely removed. Attached one-line
patch does this.

Regards,

-- 
Jeroen Schot
schot_AT_a-eskwadraat.nl (mail & jabber)
http://schot.a-eskwadraat.nl