libavfilter: Fix hang when linking with VS 2019 x64

If nasm is used to build an 'empty' assembly file, it still outputs an object file for it but the MSVC linker seems to hang when linking to it. This only happens for the v360 filter because the assembly helpers are wrapped inside %if HAVE_AVX2_EXTERNAL.

Remove that if to fix the hang, and also because the if is wrong. The functions should always be compiled and they will be plugged in at runtime if AVX2 is found. That's exactly what ff_v360_init_x86() inside vf_v360_init.c already does.

Reproduced with:

Microsoft (R) Incremental Linker Version 14.26.28806.0
NASM version 2.14.02 compiled on Dec 26 2018
Microsoft (R) C/C++ Optimizing Compiler Version 19.26.28806 for x64