libpng: Upgrade to 1.6.26

This release contains many minor and important fixes.

Libpng 1.6.26 released on October 20, 2016

Change-Id: If647f42a1fb4d7022286010d262d289dac4030df
Signed-off-by: Alex Naidis <alex.naidis@linux.com>
diff --git a/pngconf.h b/pngconf.h
index 08b4239..5c891eb 100644
--- a/pngconf.h
+++ b/pngconf.h
@@ -1,7 +1,7 @@
 
 /* pngconf.h - machine configurable file for libpng
  *
- * libpng version 1.6.25, September 1, 2016
+ * libpng version 1.6.26, October 20, 2016
  *
  * Copyright (c) 1998-2002,2004,2006-2016 Glenn Randers-Pehrson
  * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
@@ -507,9 +507,9 @@
 #  error "libpng requires a signed 32-bit (or more) type"
 #endif
 
-#if UINT_MAX > 4294967294
+#if UINT_MAX > 4294967294U
    typedef unsigned int png_uint_32;
-#elif ULONG_MAX > 4294967294
+#elif ULONG_MAX > 4294967294U
    typedef unsigned long int png_uint_32;
 #else
 #  error "libpng requires an unsigned 32-bit (or more) type"