Use symbol versioning

It should be adapted to whatever symbols are exported in the next release.

It avoid crashes if, for some reason, a process (indirectly) ends with both libmlt5.so and libmlt6.so in its image.
It also lets RPM automatically add the minimum required version for dependent packages. So, if kdenlive uses a symbol added in MLT 0.9.0 RPM will specify MLT >= 0.9.0 as a dependency. Even if kdenlive is actually compiled using MLT 0.9.2.

But it requires discipline. In the future, every new symbol exported must be added to the version script file.