MacOS' linker dislikes local static variables that are used
in two separate libraries. Removing them from the header files
fixes all the warnings.

Bug b/chromium:907088

Change-Id: I7b8ed44bf9a3180489a7407980740fd3f3863046
Reviewed-on: https://swiftshader-review.googlesource.com/c/22889Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>