pool: Install SQL schemas from src/pool

This allows us to install the schemas if either the attr-sql or sql
plugin is enabled, since both use the same schema (at least in parts).

src/Makefile.am

@@ -98,6 +98,10 @@ endif
 
 if USE_ATTR_SQL
   SUBDIRS += pool
+else
+if USE_SQL
+  SUBDIRS += pool
+endif
 endif
 
 if USE_TKM