python: -Wformat is needed by gcc 4.8

python: -Wformat is needed by gcc 4.8

This fixes errors in packages using python( build with gcc 4.8)

|
/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/sysroots/qemuarm/usr/include/python2.7/modsupport.h:27:1:
error: 'PyArg_ParseTuple' is an unrecognized format function type
[-Werror=format=]
|  PyAPI_FUNC(int) PyArg_ParseTuple(PyObject *, const char *, ...)
Py_FORMAT_PARSETUPLE(PyArg_ParseTuple, 2, 3);
|  ^
| cc1: all warnings being treated as errors
| cc1: all warnings being treated as errors

(From OE-Core rev: 5745a482a85c064a1eec960aff104cf8ce588e30)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>