mel: remove *_MIRROR from SRC_URI vardeps

Changing one of the _MIRROR variables (e.g. APACHE_MIRROR) only changes where
we're downloading something from, not what we're downloading. We don't want to
force a refetch/rebuild when the mirror variables change, so exclude it from
the SRC_URI vardeps.

JIRA: SB-2728

Signed-off-by: Christopher Larson <kergoth@gmail.com>
(cherry picked from commit efc648cd3d8151f393cdfa79cdbb750224c40c9a)
Merge pull request #176 from kergoth/sb-2728

mel: remove *_MIRROR from SRC_URI vardeps