Also added support for libxml2 in the new dash demuxer.

This adds in any missing symbols that are required due to the use of
"Dead Code Elimination" and thus prevents errors in debug builds due to
unresolved symbols.

Fixes #2

This means that librtmp is no longer required as ffmpegs internal rtmp
code is now used instead. libgmp is now a direct dependency as it is
used for rtmp crypt functions.