ARM files missing from tarball

libtheora-1.2.0.tar.gz fails to build on 32-bit ARM because two source files, lib/arm/armenc.c and lib/arm/armloop.s, are missing from the tarball.

Both of these need adding to EXTRA_DIST in lib/Makefile.am.

(I'm not sure why this hasn't affected previous releases, but maybe the tarball was built by hand before rather than using make dist?)