selftests/lib: set printf.sh executable

Test for test_printf module fails always because the test program,
printf.sh, has no execution permission.  This commit adds execution
permission to it.
Signed-off-by: NSeongJae Park <sj38.park@gmail.com>
Acked-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>