Fix Windows gdb build failure with Python 2 support

Fix Windows gdb build failure with Python 2 support

GDB fails to build for Windows host with Python 2 support enabled due
to PyFile_FromString's second argument being of type char * and being
passed a (const) string literal. This parameter is input only so this
commit fixes the issue by casting to char *.

2017-04-06  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gdb/
	* python/python.c (python_run_simple_file): Cast mode literal to
	non-const char pointer as expected by PyFile_FromString.