Missed libgc5-dev in Build-Depends

[Matthias Klose]

Pavel Epifanov Y2 writes:
 > I refer to gcc 2.95.2-13.1 package.
 > 
 > It looks like that libobjc required some header files (gc.h,...) from
 > libgc5-dev package in Potato.

in gobjc-2.95.[34] gobjc recommends libgc5-dev.