trivial: remove warnings in pfilter.c

In this commit:

   40b87d1c

 trivial: remove debug prints from pfilter

I made an error. By defining pfilter_dbg to nothing the arguments remained,
a a parens-enclosed comma expression with no effects. This fixes the
mishap and the related compiler warnings.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>