Fix make distcheck (against release 5.1.1)

Signed-off-by: Florian Forster <octo@collectd.org>
make distcheck : perllocal.pod was missing. Fixed

Signed-off-by: Florian Forster <octo@collectd.org>
src/Makefile.am: Fix "make distcheck".

protoc-c has a weird handling of files not in the CWD: They have to be
specified with the "-I" option _and_ you need to specify the full path
to the file.