tools/liblockdep: Add include directory to allow tests to compile

All of the programs in the tests directory require the
liblockdep/mutex.h header in order to compile. Add the include directory
to the compiler options so that the tests can be built with the provided
Makefile.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>