build: fix compilation of svq1enc_mmx.c with --disable-mmx

It's needed for ff_svq1enc_init_x86() even if simd functions are disabled.

Alternatively, svq1enc_init.c could be made and the relevant code moved there.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>