dsp/lossless: Average2, make a constant unsigned
dsp/lossless: Average2, make a constant unsigned

use 'u' rather than the unnecessary 'l' as a suffix. this prevents a
conversion warning with some toolchains

Change-Id: I21c33ce08819b3c839c75e03a8f7f3a6041d0695

Aexyn/libwebp,PKRoma/libwebp,redengineer/libwebp,Aexyn/libwebp,PKRoma/libwebp,PKRoma/libwebp,ttyangf/libwebp,Aexyn/libwebp,redengineer/libwebp,webmproject/libwebp,Aexyn/libwebp,ttyangf/libwebp,redengineer/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,redengineer/libwebp,ttyangf/libwebp,Aexyn/libwebp,webmproject/libwebp,redengineer/libwebp