audio/webrtc-audio-processing: unbreak on armv6 after r557409

In file included from webrtc/third_party/pffft/src/pffft.c:155:
/usr/lib/clang/11.0.0/include/arm_neon.h:32:2: error: "NEON support not enabled"
#error "NEON support not enabled"

^

webrtc/third_party/pffft/src/pffft.c:156:9: error: unknown type name 'float32x4_t'
typedef float32x4_t v4sf;

^