Fix line continuation handling for pro files in release.pl.

Due to broken handling of '\' continuation lines in pro files
the tar balls were lacking some files which were added to the
HEADER field of mumble.pro with continuations.

This patch fixes the corresponding regex in release.pl to properly
remove the continuations. It also introduces a pro file blacklist
as the fix makes previously invisible files visible to the scripts
which weren't meant to be processed.