x86/fladsp: add missing check to ff_flacdsp_init_x86()

Fixes compilation with flac decoder disabled and encoder enabled

Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
