[balsa] print-object: BalsaPrintObject is abstract

[Peter Bloomfield <peterb src gnome org>]

commit aa25187ea1e4e31055e04cf494792865e7ed1e75
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Sat May 23 10:36:32 2020 -0400

    print-object: BalsaPrintObject is abstract
    
    Define it as such, to show that it cannot be instantiated.

 src/balsa-print-object.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/balsa-print-object.c b/src/balsa-print-object.c
index a223c6fd2..3ab55591e 100644
--- a/src/balsa-print-object.c
+++ b/src/balsa-print-object.c
@@ -42,7 +42,7 @@ typedef struct {
     BalsaPrintRect rect;
 } BalsaPrintObjectPrivate;
 
-G_DEFINE_TYPE_WITH_PRIVATE(BalsaPrintObject, balsa_print_object, G_TYPE_OBJECT)
+G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE(BalsaPrintObject, balsa_print_object, G_TYPE_OBJECT)
 
 
 /* object related functions */