Update Makefile

As checked out this didn't compile for me.  As per the suggestion at http://unix.stackexchange.com/questions/18143/how-to-include-local-header-files-in-linux-kernel-module , changing the EXTRA_CFLAGS to use $(src) rather than $(PWD) corrects it for me.  (Now, it won't compile because this machine doesn't have enough memory :) )