selftests/exec: Makefile is a run-time dependency, add it to the install list

The execveat test try to exec the Makefile file and expect an EACCES
results. When running the test in the installed destination it would
fail with ENOENT since the file is not there.

Add Makefile to the TEST_FILES list so it's copied at install time.

Signed-off-by: Yannick Brosseau <scientist@fb.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>