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>

libavcodec/x86/flacdsp_init.c

@@ -38,7 +38,7 @@ av_cold void ff_flacdsp_init_x86(FLACDSPContext *c, enum AVSampleFormat fmt,
             c->lpc = ff_flac_lpc_32_sse4;
     }
     if (EXTERNAL_XOP(cpu_flags)) {
-        if (bps > 16)
+        if (bps > 16 && CONFIG_FLAC_DECODER)
             c->lpc = ff_flac_lpc_32_xop;
     }
 #endif