Clang16 does not allow implicit function declarations by default.

See also: https://wiki.gentoo.org/wiki/Modern_C_porting
Bug: https://bugs.gentoo.org/875764

Signed-off-by: Pascal Jäger <pascal.jaeger@leimstift.de>

--- a/src/main.c
+++ b/src/main.c
@@ -49,6 +49,9 @@ GList* edge_list = NULL;
 MouseState mouse_state = IDLE;
 Vertex last_click;
 
+/* Function prototypes */
+void redraw_render_area( GtkWidget* widget, GdkEventExpose* event, gpointer user_data );
+
 /* Functionality and callback functions */
 void clear_edge_list()
 {
@@ -165,7 +168,7 @@ void draw_pixbuf_to_drawing_area( GdkPixbuf* src, GtkWidget* dest )
 			 GDK_RGB_DITHER_NONE, 0, 0 );
 }
 
-gboolean redraw_render_area( GtkWidget* widget, GdkEventExpose* event, gpointer user_data )
+void redraw_render_area( GtkWidget* widget, GdkEventExpose* event, gpointer user_data )
 {
 	GtkWidget* render_drawing_area = glade_xml_get_widget( xml, "render_drawing_area" );
 	draw_pixbuf_to_drawing_area( render_result, render_drawing_area );
--- a/src/graph.c
+++ b/src/graph.c
@@ -64,6 +64,7 @@ gboolean save_graph_to_file( GList* edges, char* filename )
 	}
 
 	fclose( f );
+	return FALSE;
 }
 
 GList* load_graph_from_file( char* filename )