python plugin: fix building on OSX

python plugin: fix building on OSX

Fixes #1852

duplicate symbol _ValuesType in:
    .libs/python_la-python.o
    .libs/python_la-pyconfig.o
duplicate symbol _NotificationType in:
    .libs/python_la-python.o
    .libs/python_la-pyconfig.o
duplicate symbol _ConfigType in:
    .libs/python_la-python.o
    .libs/python_la-pyconfig.o
duplicate symbol _UnsignedType in:
    .libs/python_la-python.o
    .libs/python_la-pyconfig.o
duplicate symbol _SignedType in:
    .libs/python_la-python.o
    .libs/python_la-pyconfig.o
duplicate symbol _PluginDataType in:
    .libs/python_la-python.o
    .libs/python_la-pyconfig.o
duplicate symbol _ValuesType in:
    .libs/python_la-python.o
    .libs/python_la-pyvalues.o
duplicate symbol _NotificationType in:
    .libs/python_la-python.o
    .libs/python_la-pyvalues.o
duplicate symbol _ConfigType in:
    .libs/python_la-python.o
    .libs/python_la-pyvalues.o
duplicate symbol _UnsignedType in:
    .libs/python_la-python.o
    .libs/python_la-pyvalues.o
duplicate symbol _SignedType in:
    .libs/python_la-python.o
    .libs/python_la-pyvalues.o
duplicate symbol _PluginDataType in:
    .libs/python_la-python.o
    .libs/python_la-pyvalues.o
ld: 12 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[3]: *** [python.la] Error 1
make[2]: *** [all-recursive] Error 1
make[1]: *** [all] Error 2
make: *** [all-recursive] Error 1