CheckSymbolExists: Restore newline at end of test source

Refactoring in commit db76876db5 (Modules: Use new SOURCES_FROM_*
try_compile (1/2), 2022-09-26, v3.25.0-rc1~74^2~1) accidentally dropped
the final newline of the test source, which was previously provided by
the input to `configure_file`.  The C standard requires a newline at the
end of file, so add one explicitly.

Signed-off-by: Andrey Vostrikov <andrey.vostrikov@cogentembedded.com>