package/speexdsp+tremor: switch to new git repository

The original git server on git.xiph.org died, and the Xiph project has
now moved on to host their repositories on gitlab.comn instead.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>