Makefile: [fix] allow minor version releases by using wildcard expansion

Previously, incrementing the Release number in stb-tester.spec.in to
anything other than 1 would cause the srpm make target to break because
the name of the produced RPM would not match that which was hardcoded to
the "src_rpm" variable.

This commit fixes the issue by reducing the "src_rpm" variable to just the
suffix (which is constant) and using make target expansion and wildcards.

Fixes #241.
Yup sorry I hadn't been testing the `make rpm` target because I'd been avoiding it due to the `sudo` requirement for `yum-builddep`.

I'd also avoided the additional sub-in to stb-tester.spec.in because I didn't like the idea of updating the Makefile for each minor release (just that it seems a strange place to update) but I suppose it's not of great significance. I'll see what I can do.