[protobuf_cpp] fixed runtime issues

[goffi-contrib]

two issues were causing troubles with protobuf during runtime:

• a missing __init__.py in site-packages/google was preventing to import the lib (at least with Python 2)
• the links options (-l …) where not taken into account. By moving
  them to LDFLAGS it works correctly.

[AndreMiras]

Thanks for the PR I'd like to test it at runtime at least on Python3 before merging

[AndreMiras]

I gave it a try prior the fix. With the following import code:

from google.protobuf.pyext import _message

And I got the following runtime error:

03-16 20:17:40.227 20010 20322 I python  : Traceback (most recent call last):                                                                                             
03-16 20:17:40.227 20010 20322 I python  :   File "/home/andre/workspace/python-for-android/issues/pr1757/.buildozer/android/app/main.py", line 3, in <module>            
03-16 20:17:40.227 20010 20322 I python  : ImportError: dlopen failed: cannot locate symbol "_ZNSs4_Rep20_S_empty_rep_storageE" referenced by "/data/data/org.test.myapp/files/app/_python_bundle/site-packages/google/protobuf/pyext/_message.so"...
03-16 20:17:40.227 20010 20322 I python  : Python for android ended. 

[AndreMiras]

Thanks!
I've tested the fix on Python3 target and I confirm the linker flags update also fixes the runtime on Python3