RedHat RPM spec: updated for 5.3.0

Signed-off-by: Florian Forster <octo@collectd.org>
Build system: Don't ship the generated *.pb-c.[ch] files.

They depend on specific versions of the protobuf-c.h header file, so
shipping this will cause problems for people having a different version
of protoc-c installed. This reverts some of the work done in #269, but
distcheck should still work. Meh.
src/Makefile.am: Add *.proto to EXTRA_DIST. D'oh!