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: I59255889740195ec811a947a7130ae0918ea4b4d
diff --git a/configure.ac b/configure.ac
index ac887a0..f69c78d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -441,4 +441,5 @@
 	Doxyfile.coding
 	Doxyfile.gb
 	Doxyfile.ctrl
-	Makefile)
+	Makefile
+	contrib/libosmocore.spec)