selftests/vDSO: add additional binaries to .gitignore

Add the test binaries introduced by commit 693f5ca0 ("kselftest:
Extend vDSO selftest"), commit 03f55c79 ("kselftest: Extend vDSO
selftest to clock_getres") and commit c7e5789b ("kselftest: Move
test_vdso to the vDSO test suite") to .gitignore.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>