[Midnightbsd-cvs] mports: Makefile: update after png bump

[laffer1 at midnightbsd.org]

Log Message:
-----------
update after png bump

Modified Files:
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    mports/graphics/graphviz:
        Makefile (r1.19 -> r1.20)
        (http://cvsweb.midnightbsd.org/mports/graphics/graphviz/Makefile?r1=1.19&r2=1.20)