Add comment for gcc compiler bug work-around for SK_COMPILE_ASSERT.

If this comment had existed previously, I could have avoided breaking
some bots.

R=bsalomon@google.com

Review URL: https://codereview.chromium.org/196473013

git-svn-id: http://skia.googlecode.com/svn/trunk@13793 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed