Add version information to distribution tarball.

We somehow forgot this when adding Git version string support in commit
c5e4187590.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

Makefile.am

@@ -11,3 +11,6 @@ EXTRA_DIST = \
 BUILT_SOURCES = $(top_srcdir)/.version
 $(top_srcdir)/.version:
 	echo $(VERSION) > $@-t && mv $@-t $@
+
+dist-hook:
+	echo $(VERSION) > $(distdir)/.version