src/libpcp/getdate.y: fix compilation warnings from isalpha() et al
src/libpcp/getdate.y: fix compilation warnings from isalpha() et al

Arg should be an int, not a char.  Found on NetBSD (vm09).

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp