[gdk-pixbuf] io-png.c: Fix GCCism

[Chun-wei Fan <fanchunwei src gnome org>]

commit a1608eff6cabeac34d47ce9f3f23f7c032f493ce
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Tue Sep 15 15:00:54 2015 +0800

    io-png.c: Fix GCCism
    
    Commit 8714ab4 introduced a GCCism by doing pointer arithmetic on a
    gpointer, which is not allowed by other compilers.  Fix that by
    casting the pointer used in the addition.

 gdk-pixbuf/io-png.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gdk-pixbuf/io-png.c b/gdk-pixbuf/io-png.c
index 5690875..e5ab22b 100644
--- a/gdk-pixbuf/io-png.c
+++ b/gdk-pixbuf/io-png.c
@@ -327,7 +327,7 @@ gdk_pixbuf__png_image_load (FILE *f, GError **error)
 
        rows = g_new (png_bytep, h);
 
-        for (i = 0, ptr = pixbuf->pixels; i < h; i++, ptr += pixbuf->rowstride)
+        for (i = 0, ptr = pixbuf->pixels; i < h; i++, ptr = (guchar *) ptr + pixbuf->rowstride)
                rows[i] = ptr;
 
        png_read_image (png_ptr, rows);