swr: don't forget to link AVX/AVX2 against pthreads

swr: don't forget to link AVX/AVX2 against pthreads

Seems like the backends have been using pthreads since day one, yet
we've been missing the link.

With later commit we'll fix a typo, hence the libraries will be build
with -Wl,no-undefined, aka failing the build on unresolved symbols.

v2: Split from a larger patch.

Cc: mesa-stable@lists.freedesktop.org
Cc: Bruce Cherniak <bruce.cherniak@intel.com>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Cc: Laurent Carlier <lordheavym@gmail.com>
Fixes: c6e67f5a9373e916a8d2 "gallium/swr: add OpenSWR rasterizer"
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>