KVM: fix Kconfig menu text for -Werror

This was evidently copy and pasted from the i915 driver, but the text
wasn't updated.

Fixes: 4f337faf ("KVM: allow disabling -Werror")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>