misc: Remove gsm_subscriber.h from the db.h file

Reduce the dependencies on the header files.
diff --git a/openbsc/tests/db/db_test.c b/openbsc/tests/db/db_test.c
index f168acb..c14dd66 100644
--- a/openbsc/tests/db/db_test.c
+++ b/openbsc/tests/db/db_test.c
@@ -19,6 +19,7 @@
  */
 
 #include <openbsc/db.h>
+#include <openbsc/gsm_subscriber.h>
 
 #include <stdio.h>
 #include <string.h>