Add (init) symbol names prefixed with an underscore

The command: `python -c 'from tensorflow.contrib import tensorrt as trt'` fails
with:

File "/xxx/lib/python2.7/site-packages/tensorflow/contrib/tensorrt/wrap_conversion.py", line 24, in swig_import_helper
    _mod = imp.load_module('_wrap_conversion', fp, pathname, description)
ImportError: dynamic module does not define init function (init_wrap_conversion)

➜  1534993632 nm /xxx/lib/python2.7/site-packages/tensorflow/contrib/tensorrt/_wrap_conversion.so  | grep wrap_conversion
0000000000003670 t _init_wrap_conversion <---------- symbol defined, but local, not exposed
                 U init_wrap_conversion
                 I init_wrap_conversion (indirect for init_wrap_conversion)

Happens because the linker version script has: `init_wrap_conversion` instead of `_init_wrap_conversion`

xref: https://github.com/tensorflow/tensorflow/issues/21818
xref: https://stackoverflow.com/a/37534357