[Bug-plotutils] png-1.5 build fix

[Thomas Klausner]

png-1.5 hides structure members from public view. For this reason,
some code doesn't compile any more.

The attached patch against plotutils-2.6 fixes the problem.
I didn't find SVN or similar to check if the problem is still there.

Also, the home page at http://www.gnu.org/software/plotutils/ mentions
that 2.4.1 is the newest version and the linked manual is for 2.2.

Cheers,
 Thomas

patch-aa
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