mkosi: define ext_man macro in suse rpm build

The suse spec now uses:

 %{_mandir}/man3/*.3%{?ext_man}

which defaults to .gz and fails, as we disable compression.
Redefine it to avoid a build failure:

 Processing files: systemd-doc-260.2-00.noarch
 error: File not found: /var/tmp/BUILD/systemd-260.2-build/BUILDROOT/usr/share/man/man3/*.3.gz

mkosi/mkosi.images/build/mkosi.conf.d/opensuse/mkosi.build.chroot

@@ -95,6 +95,7 @@ build() {
         $( ((MESON_VERBOSE)) || echo "--undefine=__meson_verbose") \
         --define "meson_extra_configure_options $MKOSI_MESON_OPTIONS $MESON_OPTIONS" \
         --define "__os_install_post /usr/lib/rpm/brp-suse %{nil}" \
+        --define "ext_man %{nil}" \
         --define "__elf_exclude_path ^/usr/lib/systemd/tests/unit-tests/.*$" \
         --define "__script_requires %{nil}" \
         --define "_find_debuginfo_dwz_opts %{nil}" \