Fix deprecated G_TYPE_INSTANCE_GET_PRIVATE and G_INLINE_FUNC

with the function/declaration that they substituted.
Note that this doesn't use the recommended new GObject creation macros
because the class names in libgnucash/engine don't follow the gnome
naming convention.

libgnucash/app-utils/gnc-ui-util.c

@@ -2088,9 +2088,7 @@ typedef enum
 
 #define done_state(state) (((state) == DONE_ST) || ((state) == NO_NUM_ST))
 
-G_INLINE_FUNC long long int multiplier (int num_decimals);
-
-long long int
+static inline long long int
 multiplier (int num_decimals)
 {
     switch (num_decimals)