build: libtracker-miners-common should be a static library

This library is API-unstable helper code. In the Autotools build system we only linked to it statically and didn't install it.

Fixes a regression introduced in 8c1ff9cf, where we stopped installing the library but didn't make it static, which totally broke installed versions of Tracker.

See also: #41 (closed)

Corresponds with: tracker!101 (merged)