Only set accelerated arm fft functions if fft is enabled.

Fixes lavc compilation (linking) for configurations without fft.

Reported-by: tyler wear
Tested-by: Gavin Kinsey

libavcodec/arm/fft_fixed_init_arm.c

@@ -33,7 +33,9 @@ av_cold void ff_fft_fixed_init_arm(FFTContext *s)
 
     if (have_neon(cpu_flags)) {
         s->fft_permutation = FF_FFT_PERM_SWAP_LSBS;
+#if CONFIG_FFT
         s->fft_calc        = ff_fft_fixed_calc_neon;
+#endif
 
 #if CONFIG_MDCT
         if (!s->inverse && s->nbits >= 3) {

libavcodec/arm/fft_init_arm.c

@@ -43,8 +43,10 @@ av_cold void ff_fft_init_arm(FFTContext *s)
     int cpu_flags = av_get_cpu_flags();
 
     if (have_neon(cpu_flags)) {
+#if CONFIG_FFT
         s->fft_permute  = ff_fft_permute_neon;
         s->fft_calc     = ff_fft_calc_neon;
+#endif
 #if CONFIG_MDCT
         s->imdct_calc   = ff_imdct_calc_neon;
         s->imdct_half   = ff_imdct_half_neon;