MFH: r527232

Add libarc to LIB_DEPENDS for audio/timidity++

Up until now, libarc was only being included in LIB_DEPENDS when
the Makefile of audio/timidity++ was being used by one of its slave ports.
audio/timidity++, however, may be also used as a standalone port in which
case it needs libarc to be available.

Reported by: hselasky

Approved by: portmgr blanket (runtime fix)