fix libpng package name

Author: jcupitt

.snapcraft.yaml

@@ -49,7 +49,7 @@ parts:
         -    libexif-dev
         -    libgsf-1-dev
         -    liblcms2-dev
-        -    libpng-dev
+        -    libpng16-dev
         -    libpango1.0-dev
         -    liborc-0.4-dev
         -    libwebp-dev