multimedia/handbrake: Make Python 2.7 build dependency explicit

Handbrake has a hard build dependency on Python 2.7 and calls python2
during the build. However lang/python2 might not be installed in the
build environment (e.g. with OPTIONS_UNSET=CUPS).

python2 /wrkdirs/usr/ports/multimedia/handbrake/work/HandBrake-1.0.7/build/../gtk/src/makedeps.py
gmake[4]: python2: Command not found
gmake[4]: *** [Makefile:866: widget.deps] Error 127
gmake[4]: Leaving directory '/wrkdirs/usr/ports/multimedia/handbrake/work/HandBrake-1.0.7/build/gtk/src'