lavc/fft_template: Do not define unneded functions with --enable-small.

lavc/fft_template: Do not define unneded functions with --enable-small.

Fixes the following warnings:
In file included from libavcodec/fft_fixed.c:21:0:
libavcodec/fft_template.c:528:6: warning: ‘pass_big’ defined but not used [-Wunused-function]
 PASS(pass_big)
      ^
libavcodec/fft_template.c:505:13: note: in definition of macro ‘PASS’
 static void name(FFTComplex *z, const FFTSample *wre, unsigned int n)\
             ^~~~
CC      libavcodec/ffv1.o
In file included from libavcodec/fft_float.c:21:0:
libavcodec/fft_template.c:528:6: warning: ‘pass_big’ defined but not used [-Wunused-function]
 PASS(pass_big)
      ^
libavcodec/fft_template.c:505:13: note: in definition of macro ‘PASS’
 static void name(FFTComplex *z, const FFTSample *wre, unsigned int n)\
             ^~~~