Packaging: fix "make rpm"

[pavoldomin]

After the introduction of read-only variables (#710), make rpm fails with:

...
== Building RPM package rear--git201512081908 ==
rpmbuild -tb --clean \
                --define "_rpmfilename %%{NAME}-%%{VERSION}-%%{RELEASE}.%%{ARCH}.rpm" \
                --define "debug_package %{nil}" \
                --define "_rpmdir %(pwd)" rear--git201512081908.tar.gz
error: line 11: Empty tag: Version:
make: *** [rpm] Error 1

This patch addresses this issue.

[jsmeix]

@pavoldomin
many thanks for spotting and fixing that unexpected
regression of my #710
and in general many thanks for all your valuable
contribution to Relax-and-Recover!