libtorrent-rasterbar: Use Boost supporting Python

Since 772eef91

, the Boost package
doesn't support Python by default anymore, so we need to override it
with Python support because --enable-python-binding is enabled by
default in libtorrent-rasterbar.

The build of the package now succeeds again on my x86_64-linux machine.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @phreedom