contrib: integrate RPM spec

Remove OpenSUSE bug report link, set version to @VERSION@, make it build
with CentOS 8 etc.

Related: OS#4550
Change-Id: I1d03ac87a7d0c3c600d187f3e485cb2dab8838bb
diff --git a/configure.ac b/configure.ac
index 349c8db..7c63437 100644
--- a/configure.ac
+++ b/configure.ac
@@ -209,4 +209,5 @@
     doc/manuals/Makefile
     contrib/Makefile
     contrib/systemd/Makefile
+    contrib/osmo-mgw.spec
     Makefile)