DatabaseGlue: add function GetDatabase()

Allow using the object outside of DatabaseGlue.cxx.

src/DatabaseGlue.cxx

@@ -18,6 +18,7 @@
  */
 
 #include "config.h"
+#include "DatabaseGlue.hxx"
 
 extern "C" {
 #include "database.h"
@@ -79,6 +80,14 @@ db_finish(void)
 		delete db;
 }
 
+const Database *
+GetDatabase()
+{
+	assert(db == NULL || db_is_open);
+
+	return db;
+}
+
 struct directory *
 db_get_root(void)
 {