avcodec/x86/dcadsp_init: fix compilation failure without FMA3

alternatively the call could be put under #if or the #if
over the function removed
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/x86/dcadsp_init.c

@@ -82,6 +82,10 @@ SYNTH_FILTER_FUNC(sse)
 #endif
 SYNTH_FILTER_FUNC(sse2)
 SYNTH_FILTER_FUNC(avx)
+static void synth_filter_fma3(FFTContext *imdct,
+                             float *synth_buf_ptr, int *synth_buf_offset,
+                             float synth_buf2[32], const float window[512],
+                             float out[32], const float in[32], float scale);
 #if HAVE_FMA3_EXTERNAL
 SYNTH_FILTER_FUNC(fma3)
 #endif