Deparse: Don’t parenthesize \my @a needlessly

because \my(@a) is different, and produces a list of element refs,
rather than a single reference to the array.

No good:

$ ./perl -Ilib -MO=Deparse -e '\my @x'
\my(@x);
-e syntax OK
deparse-skips.txt: coreamp.t now passes
deparse-skips.txt: crypt.t now passes
deparse-skips.txt: do.t now passes