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/pngstruct.h b/pngstruct.h
index 2b0eb49..55516ea 100644
--- a/pngstruct.h
+++ b/pngstruct.h
@@ -263,7 +263,7 @@
                               /* pixel depth used for the row buffers */
    png_byte transformed_pixel_depth;
                               /* pixel depth after read/write transforms */
-#if PNG_ZLIB_VERNUM >= 0x1240
+#if ZLIB_VERNUM >= 0x1240
    png_byte zstream_start;    /* at start of an input zlib stream */
 #endif /* Zlib >= 1.2.4 */
 #if defined(PNG_READ_FILLER_SUPPORTED) || defined(PNG_WRITE_FILLER_SUPPORTED)