install hlr.sql in prefix/doc/osmo-hlr/

In particular I need this to start a fresh osmo-hlr instance on the
osmo-gsm-tester. Might also come in handy during packaging?

Change-Id: I08e48375814ab93691892299d34909c6d0bf12a2
diff --git a/sql/Makefile.am b/sql/Makefile.am
index 059a571..ab44e2a 100644
--- a/sql/Makefile.am
+++ b/sql/Makefile.am
@@ -2,3 +2,6 @@
 	hlr_data.sql \
 	hlr.sql \
 	$(NULL)
+
+docsdir = $(datadir)/doc/osmo-hlr
+docs_DATA = $(srcdir)/hlr.sql