Specify that the shell to be used on src_tinc is bash

This fixes building src_tinc on make 3.81-3.82. This GNU Make issue describes the problem:

http://savannah.gnu.org/bugs/?30614

The lines like the following were failing because of make defaulting to /bin/sh:

```
	@cd tinc && autoreconf -fsi &>> $(LOG_FILE) && \
```