[retro-gtk] gl-display: Fix build warning message

[Adrien Plazas <aplazas src gnome org>]

commit 033da0e36e6e0e9530d993a665e3662925e9dfd7
Author: RoubyPA <pa rouby gmail com>
Date:   Wed Feb 14 11:37:04 2018 +0100

    gl-display: Fix build warning message
    
    Fix build warning message in retro-gtk/retro-gl-display.c

 retro-gtk/retro-gl-display.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/retro-gtk/retro-gl-display.c b/retro-gtk/retro-gl-display.c
index 7dfbd6c..da353b3 100644
--- a/retro-gtk/retro-gl-display.c
+++ b/retro-gtk/retro-gl-display.c
@@ -261,7 +261,7 @@ retro_gl_display_realize (RetroGLDisplay *self)
                                              GL_FLOAT,
                                              GL_FALSE,
                                              sizeof (RetroVertex),
-                                             offsetof (RetroVertex, position));
+                                             (const GLvoid *) offsetof (RetroVertex, position));
 
     retro_glsl_filter_set_attribute_pointer (self->glsl_filter[filter],
                                              "texCoord",
@@ -269,7 +269,7 @@ retro_gl_display_realize (RetroGLDisplay *self)
                                              GL_FLOAT,
                                              GL_FALSE,
                                              sizeof (RetroVertex),
-                                             offsetof (RetroVertex, texture_coordinates));
+                                             (const GLvoid *) offsetof (RetroVertex, texture_coordinates));
   }
 
   glDeleteFramebuffers (1, &self->framebuffer);