In your code, the first step is to #include the needed headers: depending on your header include strategy, this can be as simple as #include "maman-bar.h" or as complicated as tens of #include lines ending with #include "maman-bar.h":

/*
 * Copyright information
 */

#include "maman-bar.h"

/* If you use Pimpls, include the private structure 
 * definition here. Some people create a maman-bar-private.h header
 * which is included by the maman-bar.c file and which contains the
 * definition for this private structure.
 */
struct _MamanBarPrivate {
  int member_1;
  /* stuff */
};

/* 
 * forward definitions
 */

Implement maman_bar_get_type and make sure the code compiles:

GType
maman_bar_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanBarClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      NULL,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (MamanBar),
      0,      /* n_preallocs */
      NULL    /* instance_init */
      };
      type = g_type_register_static (G_TYPE_OBJECT,
                                     "MamanBarType",
                                     &info, 0);
    }
    return type;
}