attempt to fix RPM spec file after recent soversion bump

In I64ff22193ab2a95a9a7d66e9957a875d096e23de the soversion was
chhanged, but the spec.in file was not adjusted accordingly.

Change-Id: I45d7321adb2db8e7b9ea41b6bcf0c6cc59984a0e
diff --git a/contrib/osmo-mgw.spec.in b/contrib/osmo-mgw.spec.in
index efb166a..ee85db3 100644
--- a/contrib/osmo-mgw.spec.in
+++ b/contrib/osmo-mgw.spec.in
@@ -120,7 +120,7 @@
 %config(noreplace) %{_sysconfdir}/osmocom/osmo-mgw.cfg
 
 %files -n libosmo-mgcp-client8
-%{_libdir}/libosmo-mgcp-client.so.6*
+%{_libdir}/libosmo-mgcp-client.so.8*
 
 %files -n libosmo-mgcp-client-devel
 %{_libdir}/libosmo-mgcp-client.so