test/parser: Add dependency on libpbcore.la

test/parser: Add dependency on libpbcore.la

Currently, build only a test object will fail:

  [jk@pablo obj]$ make ./test/parser/test-grub2-single

    CCLD     test/parser/test-grub2-single
  libtool: link: cannot find the library `lib/libpbcore.la' or unhandled argument `lib/libpbcore.la'

We're adding this to the link argument, but not as a dependency. This
change adds the dependency.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>