patch 8.2.1878: GTK: error for redefining function

Problem:    GTK: error for redefining function. (Tony Mechelynck)
Solution:   Remove "gtk_" prefix from local functions and prepend "gui_" to
            global functions.
diff --git a/src/gui_gtk.c b/src/gui_gtk.c
index 1733048..48910b2 100644
--- a/src/gui_gtk.c
+++ b/src/gui_gtk.c
@@ -810,7 +810,7 @@
     void
 gui_mch_set_text_area_pos(int x, int y, int w, int h)
 {
-    gtk_form_move_resize(GTK_FORM(gui.formwin), gui.drawarea, x, y, w, h);
+    gui_gtk_form_move_resize(GTK_FORM(gui.formwin), gui.drawarea, x, y, w, h);
 }
 
 
@@ -1005,7 +1005,7 @@
 gui_mch_set_scrollbar_pos(scrollbar_T *sb, int x, int y, int w, int h)
 {
     if (sb->id != NULL)
-	gtk_form_move_resize(GTK_FORM(gui.formwin), sb->id, x, y, w, h);
+	gui_gtk_form_move_resize(GTK_FORM(gui.formwin), sb->id, x, y, w, h);
 }
 
     int
@@ -1111,7 +1111,7 @@
 	GtkAdjustment *adjustment;
 
 	gtk_widget_set_can_focus(sb->id, FALSE);
-	gtk_form_put(GTK_FORM(gui.formwin), sb->id, 0, 0);
+	gui_gtk_form_put(GTK_FORM(gui.formwin), sb->id, 0, 0);
 
 	adjustment = gtk_range_get_adjustment(GTK_RANGE(sb->id));