Fix webp build to avoid linking errors when building on mac

This modifies the definitions of libwebp-decode and libwebp-encode to
match the definitions used in the upstream version of Skia. This avoids
linking errors on mac when trying to link both libraries, as it puts the
alpha processing files only in the decode library.

Test: build and link both libraries into a third shared library on mac
Change-Id: Ib2846f08e1ed6eed93bc67a031f49d0c41ff08b6
1 file changed