ANDROID: kbuild: fix UNUSED_KSYMS_WHITELIST backport

The backport of the UNUSED_KSYMS_WHITELIST feature to android-4.19
missed a dependency on $MODVERDIR (which is still there in 4.19), hence
causing it to un-export any symbol that is not on the whitelist, even if
it has an in-tree user.

I was _really_ close to call that a 'feature', but it wasn't exactly
intended, so I'll call it a 'bug' instead.

Kill the bugger.

Bug: 148277666
Fixes: 3d0431a8 ("BACKPORT: FROMLIST: kbuild: allow symbol
whitelisting with TRIM_UNUSED_KSYMS")
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: I35449dc92437f2928a659c8ecd6fbf725e0e1b87