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

Reduce the dependencies on the header files.
diff --git a/openbsc/src/db.c b/openbsc/src/db.c
index 12422fa..d672081 100644
--- a/openbsc/src/db.c
+++ b/openbsc/src/db.c
@@ -30,6 +30,7 @@
 #include <dbi/dbi.h>
 
 #include <openbsc/gsm_data.h>
+#include <openbsc/gsm_subscriber.h>
 #include <openbsc/gsm_04_11.h>
 #include <openbsc/db.h>
 #include <openbsc/debug.h>