kselftest: Fix error message for unconfigured LLVM builds

We are missing a ) when we attempt to complain about not having enough
configuration for clang, resulting in the rather inscrutable error:

../lib.mk:23: *** unterminated call to function 'error': missing ')'.  Stop.

Add the required ) so we print the message we were trying to print.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>