um: Make GCOV depend on !KCOV

[ Upstream commit 550ed0e2036663b35cec12374b835444f9c60454 ]

Both do more or less the same thing and are mutually exclusive.
If both are enabled the build will fail.
Sooner or later we can kill UML's GCOV.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>