Move config.h include from st.c to x.c

Move config.h include from st.c to x.c

config.h includes references to KeySyms and other X stuff.  Until we
come up with a cleaner way to separate configuration, it is simpler
(leads to more code removal) to have this here.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>