Add -fcommon C flag to fix compilation error

See the following issue:
TheNextLVL#3

setup.py

@@ -43,4 +43,4 @@
       url              = 'http://sourceforge.net/projects/raspberry-gpio-python/',
       classifiers      = classifiers,
       packages         = ['RPi','RPi.GPIO'],
-      ext_modules      = [Extension('RPi._GPIO', ['source/py_gpio.c', 'source/c_gpio.c', 'source/cpuinfo.c', 'source/event_gpio.c', 'source/soft_pwm.c', 'source/py_pwm.c', 'source/common.c', 'source/constants.c'])])
+      ext_modules      = [Extension('RPi._GPIO', ['source/py_gpio.c', 'source/c_gpio.c', 'source/cpuinfo.c', 'source/event_gpio.c', 'source/soft_pwm.c', 'source/py_pwm.c', 'source/common.c', 'source/constants.c'], extra_compile_args = ['-fcommon'])])