selftests: Mark ksft_exit_fail_perror() as __noreturn

Let the compilers (clang) know that this function would just call
exit() and would never return. It is needed to avoid false positive
static analysis errors. All similar functions calling exit()
unconditionally have been marked as __noreturn.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>