Log Message:
protobuf-c: fix build failure in bulk builds

This has been observed on both NetBSD and SunOS. Build fails with:
"google/protobuf/descriptor.proto: File not found."

This message comes from:
/usr/pkg/bin/protoc -I. --cpp_out=. ./protobuf-c/protobuf-c.proto

-I. is the current directory, WRKSRC.
But descriptor.proto comes from the devel/protobuf package and is not
present in WRKSRC. Fix by symlinking in include/google from the buildlink
directory under WRKSRC.