build: clang: Silence -Wcast-align

Since ExoBinding is now deprecated, there is no point in changing the code to fix these warnings.

For GdkPixbuf it seems to be false positives, see #67 (comment 50795)

For ExoIconView it could be improved, but I don't want to bother with that now and risk introducing regressions. I'm going to open a new dedicated issue.

Fixes: #67 (closed)