added tag_propagation_policy_t to block_gateway

Adding an enum tag_propagation_policy_t to the gr namespace in block_gateway.h.

This adds TPP_DONT etc to the gr. module. Therefore, self.set_tag_propagation_policy works from python now, too:

```python
from gnuradio import gr

class my_block(gr.basic_block):
    def __init__(self):
...
        self.set_tag_propagation_policy(gr.TPP_DONT)

```