Fix build with older GCC

Build with older gcc fails with:

  CC       libosinfo_1_0_la-osinfo_avatar_format.lo
In file included from ../osinfo/osinfo_os.h:30,
                 from ../osinfo/osinfo.h:55,
                 from osinfo_avatar_format.c:28:
../osinfo/osinfo_media.h:68: error: redefinition of typedef 'OsinfoMedia'
../osinfo/osinfo_install_script.h:43: error: previous declaration of 'OsinfoMedia' was here

To avoid redefinition, use forward-declaration in a similar way
like it's done for OsinfoOs in osinfo/osinfo_os.h.