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)\
             ^~~~

libavcodec/fft_template.c

@@ -523,9 +523,11 @@ static void name(FFTComplex *z, const FFTSample *wre, unsigned int n)\
 }
 
 PASS(pass)
+#if !CONFIG_SMALL
 #undef BUTTERFLIES
 #define BUTTERFLIES BUTTERFLIES_BIG
 PASS(pass_big)
+#endif
 
 #define DECL_FFT(n,n2,n4)\
 static void fft##n(FFTComplex *z)\