Fix ordering of referenced shared libraries in workgen

On newer systems, we see this kind of failure:

+ python ../bench/workgen/runner/split_stress.py
Traceback (most recent call last):
File "../bench/workgen/runner/split_stress.py", line 34, in <module>
from runner import *
File "/home/jenkins/jenkins/workspace/wiredtiger-test-split-stress/bench/workgen/runner/runner/_init_.py", line 85, in <module>
import workgen
File "/home/jenkins/jenkins/workspace/wiredtiger-test-split-stress/bench/workgen/workgen/workgen.py", line 32, in <module>
_workgen = swig_import_helper()
File "/home/jenkins/jenkins/workspace/wiredtiger-test-split-stress/bench/workgen/workgen/workgen.py", line 24, in swig_import_helper
import _workgen
ImportError: /home/jenkins/jenkins/workspace/wiredtiger-test-split-stress/build_posix/bench/workgen/_workgen.so: undefined symbol: __wt_epoch