python: Silence some warnings via -Wno-uninitialized.

Silence some warnings when building the Python bindings:

  sigrok/core/classes_wrap.cpp: In function ‘PyObject* _wrap_new_OutputFormatMap(PyObject*, PyObject*)’:
  sigrok/core/classes_wrap.cpp:5232:4: warning: ‘argv[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      res = SWIG_ConvertPtr(obj,(void**)&p,swig::type_info<map_type>(),0);
      ^
  sigrok/core/classes_wrap.cpp:14383:13: note: ‘argv[0]’ was declared here
     PyObject *argv[2];
               ^
  sigrok/core/classes_wrap.cpp: In function ‘PyObject* _wrap_new_ChannelGroupMap(PyObject*, PyObject*)’:
  sigrok/core/classes_wrap.cpp:5232:4: warning: ‘argv[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      res = SWIG_ConvertPtr(obj,(void**)&p,swig::type_info<map_type>(),0);
      ^
  sigrok/core/classes_wrap.cpp:23356:13: note: ‘argv[0]’ was declared here
     PyObject *argv[2];
               ^

We add -Wno-uninitialized since the warnings are harmless and we really
don't care about them in the generated classes_wrap.cpp.

This fixes parts of #417.