This fixes the invocation of protoc in mumble_proto.pro to be compatible with out-of-tree builds. For example, this is how Qt Creator builds Mumble by default.
For out-of-tree builds, our include path flags to protoc aren't sufficient. This is because protoc requires that the .proto file it is building is part of the include path. To fix this for out-of-tree builds, we add the absolute path to the directory that contains the .proto file.
This allows protoc to work for out-of-tree builds.
For reference, the error we got (before this commit) from protoc in out-of-tree builds was:
"File does not reside within any path specified using --proto_path (or -I). You must specify a --proto_path which encompasses this file. Note that the proto_path must be an exact prefix of the .proto file names -- protoc is too dumb to figure out when two paths (e.g. absolute and relative) are equivalent (it's harder than you think)."