icalendar.l: Refer to base64.h relative to root of source.

icalendar.l: Refer to base64.h relative to root of source.

Ken reported a missing-include-file problem when building so the product
ends up outside the source tree, e.g. running configure from elsewhere.
It must also depend on his environment, e.g. OS or compiler, because the
problem isn't reproducible with Arch Linux and gcc, and be specific to C
source built from other source files, e.g. lex(1).  However, given that
the existing sibling icalparse.h is referred to as sbr/..., duplicate
that so it builds for Ken too.