Fix silly gcc warning.

Explicitly specify the underlying type of LinkerPatch::Type
as uint8_t because gcc would select a bigger type and then
complain that
    'art::LinkerPatch::patch_type_' is too small to hold all
    values of 'enum class art::LinkerPatch::Type'
which is really ridiculous given that the enum contains only
a handful enumerators with default (small) values.

Change-Id: I4c4b21cde62e56b62488bae99d3690d900c3c29e