use absolute path for RELATIVE_BASE_DIR

[furushchev]

Fixes #36

By this PR, the first include directory for compiling .proto file is given as absolute path instead of relative path.

[BillWSY]

Thanks for the PR. Did you have a chance to test if the original example still works? If the examples in https://github.com/CogRob/catkin_grpc_examples still work, I am more than happy to accept this PR.

Again, thanks for your contribution.

[furushchev]

@BillWSY Thanks for the comment!
I tried greeter_cc_server / greeter_cc_client and greeter_py_server / greeter_py_client in catkin_grpc_examples/greeter project.
Even with/without the change of this PR, it occurs errors on running greeter_py_server and greeter_py_client:

$ rosrun greeter greeter_py_server 
Traceback (most recent call last):
  File "/home/furushchev/catkin_ws/src/catkin_grpc_examples/greeter/scripts/greeter_py_server", line 3, in <module>
    from greeter import greeter_py_server
  File "/home/furushchev/catkin_ws/src/catkin_grpc_examples/greeter/src/greeter/greeter_py_server.py", line 15, in <module>
    class Greeter(hello_pb2.GreeterServicer):
AttributeError: 'module' object has no attribute 'GreeterServicer'

After I fixed the import path (created another PR), it works with/without this PR.

[BillWSY]

Thank you so much! Apparently many things have changed since I originally made this script. I will merge the PR for now, but I will come back later to update it a bit to keep it up with the upstream.

[furushchev]

@BillWSY Thanks for merging! It'd be better if the change is released. Thank you!