Check libpng version before reading color space

TBR=scroggo@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1767723003

Review URL: https://codereview.chromium.org/1767723003
diff --git a/src/codec/SkPngCodec.cpp b/src/codec/SkPngCodec.cpp
index 1b51432..23f7bee 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -178,6 +178,8 @@
 // return NULL.
 SkColorSpace* read_color_space(png_structp png_ptr, png_infop info_ptr) {
 
+#if (PNG_LIBPNG_VER_MAJOR > 1) || (PNG_LIBPNG_VER_MAJOR == 1 && PNG_LIBPNG_VER_MINOR >= 6)
+
     // First check for an ICC profile
     png_bytep profile;
     png_uint_32 length;
@@ -249,6 +251,8 @@
         return SkColorSpace::NewRGB(toXYZD50, gammas);
     }
 
+#endif // LIBPNG >= 1.6
+
     // Finally, what should we do if there is no color space information in the PNG?
     // The specification says that this indicates "gamma is unknown" and that the
     // "color is device dependent".  I'm assuming we can represent this with NULL.