dist: include .[tarball-]version and git-version-gen files

Most of other [lib]osmo-projects do include these files in the release
tarballs.  Do the same for the sake of consistency.

Change-Id: I5c15cefb68ab787819edaa1a097fe397837e0bcd
Related: OS#6227
diff --git a/Makefile.am b/Makefile.am
index a1642d4..f2a2c31 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -11,3 +11,17 @@
 endif
 
 SUBDIRS += src tests
+
+BUILT_SOURCES = \
+	$(top_srcdir)/.version \
+	$(NULL)
+EXTRA_DIST = \
+	git-version-gen \
+	.version \
+	$(NULL)
+
+# versioning magic
+$(top_srcdir)/.version:
+	echo $(VERSION) > $@-t && mv $@-t $@
+dist-hook:
+	echo $(VERSION) > $(distdir)/.tarball-version