pw_protobuf_compiler: Add protobuf dependency

This change adds the Python protobuf library as a dependency to the
pw_protobuf_compiler module, as third-party generators such as nanopb
require it.

Change-Id: I3eb86360ab8c67bf66cb505eeb5797319ff7b87e
diff --git a/pw_protobuf_compiler/py/setup.py b/pw_protobuf_compiler/py/setup.py
index 4ca1ec7..bf706c7 100644
--- a/pw_protobuf_compiler/py/setup.py
+++ b/pw_protobuf_compiler/py/setup.py
@@ -35,6 +35,7 @@
         ['generate_protos = pw_protobuf_compiler.generate_protos:main']
     },
     install_requires=[
+        'protobuf',
         'pw_cli',
     ],
 )