sql/Makefile.am: Make docsdir completely configurable

$(docdir) defaults to $(datadir)/doc/osmo-hlr

Change-Id: I77fa16d0edcf88a8e120921504cd088328077836
diff --git a/sql/Makefile.am b/sql/Makefile.am
index ab44e2a..f71037b 100644
--- a/sql/Makefile.am
+++ b/sql/Makefile.am
@@ -3,5 +3,5 @@
 	hlr.sql \
 	$(NULL)
 
-docsdir = $(datadir)/doc/osmo-hlr
+docsdir = $(docdir)
 docs_DATA = $(srcdir)/hlr.sql