avcodec/fft: Fix "warning: unused variable"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/fft.c

@@ -189,7 +189,7 @@ av_cold int ff_fft_init(FFTContext *s, int nbits, int inverse)
         fft_perm_avx(s);
     } else {
         for(i=0; i<n; i++) {
-            int j = i;
+            j = i;
             if (s->fft_permutation == FF_FFT_PERM_SWAP_LSBS)
                 j = (j&~3) | ((j>>1)&1) | ((j<<1)&2);
             s->revtab[-split_radix_permutation(i, n, s->inverse) & (n-1)] = j;