bug#52519: glibmm-2.64 propagates two different versions of libsigc++

[Leo Famulari]

glibmm-2.64 has this:

------
     (propagated-inputs
      (modify-inputs (package-propagated-inputs glibmm)
        (prepend libsigc++-2)))))
------

And, the glibmm package also propagates libsigc++.

So:

------
$ guix show glibmm@2.64 | grep libsigc
dependencies: doxygen@1.9.1 glib@2.70.0 graphviz@2.49.0 libsigc++@2.9.3
+ libsigc++@3.0.6 libxslt@1.1.34 m4@1.4.18 mm-common@1.0.3 perl@5.34.0
------

It propagates both versions of libsigc++, and thus cannot be installed
into a profile. 1785 packages depend on glibmm-2.64.

This breaks synfig, which propagates glibmm, as requested in its 'synfig.pc'
pkg-config file.