Allow importing parse tree proto with cgo disabled

[anuraaga]

I am interested in using just the proto definition in an environment without cgo. Currently, this does not work because

• The only Go file in parser package requires cgo, and Go doesn't allow a package with no Go files
• pg_query.go imports names from parser and requires cgo

Would this change be OK to allow the proto to be used by itself without cgo required?

[lfittl]

Thanks for the contribution! - I think that seems acceptable and doesn't hurt the use case where cgo is available.

I'll push up a small adjustment to also add the same build tag to the test files in the top level folder, so running CGO_ENABLED=0 make doesn't fail. Of course it doesn't actually do anything at this point, but that could be changed in the future when there is something to test that doesn't require cgo.