cgo: silence false-positive clang warning

[JackMordaunt]

Every build with Clang spits out this annoying warning message:

# github.com/chai2010/webp
In file included from z_libwebp_src_enc_picture_csp_enc.c:9: ../../../../go/pkg/mod/github.com/chai2010/webp@v1.1.1/internal/libwebp-1.0.2/src/enc/picture_csp_enc.c:1002:40: warning: result of '2 ^ ALPHA_OFFSET' is 1; did you mean '1 << ALPHA_OFFSET' (8)? [-Wxor-used-as-pow]

This commits silences that warning by supplying the requisite CFLAG.

Signed-off-by: Jack Mordaunt jackmordaunt.dev@gmail.com

[lloyd]

👍

love it!