selftests/memfd: correct run_tests.sh permission

to fix the following issue:
------------------
TAP version 13
selftests: run_tests.sh
========================================
selftests: Warning: file run_tests.sh is not executable, correct this.
not ok 1..1 selftests: run_tests.sh [FAIL]
------------------
Signed-off-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>