loadSymbol: remove RTLD_LOCAL in dlopen

[timj]

Mario Juric notes:
Otherwise (at least on OS X) when a library opened as such is
needed later by a different library, its symbols aren't found.

In particular, this was a problem with MKL-enabled numpy on
anaconda, causing errors such as this (when building ip-diffim):

====
ImportError: dlopen(/Users/mjuric/projects/conda-lsst/miniconda/envs/_build/lib/python2.7/site-packages/scipy/linalg/_fblas.so, 2): Symbol not found: ___kmpc_atomic_cmplx4_add
  Referenced from: /Users/mjuric/projects/conda-lsst/miniconda/envs/_build/lib/python2.7/site-packages/scipy/linalg/../../../../libmkl_intel_thread.dylib
    Expected in: flat namespace
     in /Users/mjuric/projects/conda-lsst/miniconda/envs/_build/lib/python2.7/site-packages/scipy/linalg/../../../../libmkl_intel_thread.dylib
====

The problem is easy to replicate in general, by importing the
`_lsstcppimport` module, followed by importing `numpy`.

Removing RTLD_LOCAL fixes this on the Mac, and shouldn't impact Linux
because it is the default there.