kvmtool: Makefile: fix indentation of warning stanza

If a static libc is not present in the system the build will fail with
make complaining about commands starting before the first target. The
patch fixes indentation of a warning about missing static libc, thus
fixing the build.

Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>