selftests/memfd: correct run_tests.sh permission

to fix the following issue:
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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]
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Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>