debian: Bump symbol versions

This ensures we get tighter dependencies when built against
this version with new symbols.

We don't bother with individual symbols yet.

debian/libfeedback-0.0-0.symbols

 libfeedback-0.0.so.0 libfeedback-0.0-0 #MINVER#
 * Build-Depends-Package: libfeedback-dev
- *@LIBFEEDBACK_0_0_0 0.0.0+git20200305
+ *@LIBFEEDBACK_0_0_0 0.0.0+git20200707