Export pqueue functions.

04dbb7f1d185af0837d46ac76bf899df6cdd2cc5 added tests for the pqueue
functions. However, when building as a shared library, the test binary
needs access to the raw pqueue functions which require them to be
exported.

Change-Id: Iffb22fec491082ff43f06a7119560610425cf20e
Reviewed-on: https://boringssl-review.googlesource.com/1711
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>