selftests/resctrl: Create .gitignore to include resctrl_tests

Create .gitignore to hold the test file resctrl_tests generated after
compiling.

Suggested-by: Shuah Khan <skhan@linuxfoundation.org>
Tested-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>