parser: fix build issue with REALLOCARRAY check

On some systems the build of the parser is spitting out

cc: fatal error: no input files
compilation terminated.

This is being caused by the REALLOCARRAY checkfailing due to cpp trying to check for both input and output files and not correctly falling back to stdin/stdout if infile and outfile aren't specified.

Fix this by being explicit that infile and outfile are supposed to use stdin and stdout.

Signed-off-by: John Johansen john.johansen@canonical.com