selftests/Makefile: Add missed closing `"` in comment

A comment for make command usage in `selftets/Makefile` has opening `"`
but no closing `"`.  This commit adds the missed `"` in the comment.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>