[gl2ps] Missing const in gl2psPrintPostScriptDash parameter found by latest gcc

[Travis Cobbs]

The last parameter of gl2psPrintPostScriptDash should be const char *,
instead of char *.  The latest gcc spits out warnings when you use literal
strings in locations that aren't const, and gl2psPrintPostScriptDash is
called with "setdash" and "d" as the last parameter.  I made this change in
my local code, and the warning went away, and it didn't produce a compile
error.

--Travis Cobbs
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