priv/Makefile.am: Hack `libpriv' so that parallel builds work.

priv/Makefile.am: Hack `libpriv' so that parallel builds work.

The library `libpriv.a' is built here, but the `libpriv' variable is set
to `$(top_builddir)/priv/libpriv.a' instead, which doesn't match.  The
result is that a parallel build might try to link the helper before
building the library, find that the library (found via the variable)
doesn't exist, and fail.

Fix this by hardwiring the library name locally, since we know where
it's meant to be.  I'd override the variable value but Automake is
petty and prints warnings if I do that.