build: Move foward typedefs to gcal-types.h

Standard practice; gcal-types.h is meant to only be
used by header files.

src/core/gcal-manager.h

@@ -22,6 +22,7 @@
 #include "e-cal-data-model.h"
 #include "gcal-calendar.h"
 #include "gcal-event.h"
+#include "gcal-types.h"
 
 #include <libecal/libecal.h>
 
@@ -30,8 +31,6 @@ G_BEGIN_DECLS
 #define GCAL_TYPE_MANAGER (gcal_manager_get_type ())
 G_DECLARE_FINAL_TYPE (GcalManager, gcal_manager, GCAL, MANAGER, GObject)
 
-typedef struct _GcalContext GcalContext;
-
 GcalManager*         gcal_manager_new                            (GcalContext        *context);
 
 ESource*             gcal_manager_get_source                     (GcalManager        *self,

src/gcal-types.h

@@ -0,0 +1,29 @@
+/* gcal-types.h
+ *
+ * Copyright 2020 Georges Basile Stavracas Neto <georges.stavracas@gmail.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+typedef struct _GcalContext GcalContext;
+
+G_END_DECLS

src/search/gcal-search-engine.h

@@ -22,13 +22,13 @@
 
 #include <gio/gio.h>
 
+#include "gcal-types.h"
+
 G_BEGIN_DECLS
 
 #define GCAL_TYPE_SEARCH_ENGINE (gcal_search_engine_get_type())
 G_DECLARE_FINAL_TYPE (GcalSearchEngine, gcal_search_engine, GCAL, SEARCH_ENGINE, GObject)
 
-typedef struct _GcalContext GcalContext;
-
 GcalSearchEngine*    gcal_search_engine_new                      (GcalContext        *context);
 
 void                 gcal_search_engine_search                   (GcalSearchEngine   *self,