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: I824b67f2d590ac2aa9f2e4fa4387a5283cf22521
diff --git a/configure.ac b/configure.ac
index 5c0bede..9432715 100644
--- a/configure.ac
+++ b/configure.ac
@@ -273,4 +273,5 @@
     doc/manuals/Makefile
     contrib/Makefile
     contrib/systemd/Makefile
+    contrib/osmo-sgsn.spec
     Makefile)