x86/bswapdsp: add missing preprocessor wrappers for AVX2 functions

Fixes build with old nasm/yasm.
Signed-off-by: NJames Almer <jamrial@gmail.com>

libavcodec/x86/bswapdsp.asm

@@ -157,5 +157,7 @@ BSWAP32_BUF
 INIT_XMM ssse3
 BSWAP32_BUF
 
+%if HAVE_AVX2_EXTERNAL
 INIT_YMM avx2
 BSWAP32_BUF
+%endif