Included string.h as strdup requires string.h

This doesn't seem to be an issue on Mac OSX, but when I build node-MariaSQL on CentOS 6.4 x86_64, there are compile issues. 

I have fixed this issue by including string.h 

references:
http://linux.die.net/man/3/strdup
http://pubs.opengroup.org/onlinepubs/009695399/functions/strdup.html