Kbuild: make MODVERSIONS support depend on not being a compile test build

Currently the Rust support is gated on not having MODVERSIONS enabled,
and as a result an "allmodconfig" build will disable Rust build tests.

While MODVERSIONS configurations are worth build testing, the feature is
not actually meaningful unless you run the result, and I'd rather get
build coverage of Rust than MODVERSIONS. So let's disable MODVERSIONS
for build testing until the Rust side clears up.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>