gitignore: add tools/uncoded_frame

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86/mpegaudiodsp.c: msvc compilation error without sse/avx_external

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>