Unit: avoided needlessly setting lib in nxt_unit_shm_open().
Unit: avoided needlessly setting lib in nxt_unit_shm_open().

As was pointed out by the cppcheck[0] static code analysis utility, lib
was being set in nxt_unit_shm_open() regardless of platform when in fact
it's only used when (NXT_HAVE_MEMFD_CREATE || NXT_HAVE_SHM_OPEN).

Move the variable declaration & definition to be within the

  #if (NXT_HAVE_MEMFD_CREATE || NXT_HAVE_SHM_OPEN)

block.

[0]: https://cppcheck.sourceforge.io/

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