Fix a missing space after the -I path
Signed-off-by: Serge Hallyn <serge@hallyn.com>
Cherry-picked-from: 81b5b26925 ("libsubid test makefile: fix a typo")
Fixes: 6b9391b581fd ("tests/: Support run_some from exported tarball")
Signed-off-by: Alejandro Colomar <alx@kernel.org>