Add MinGW support for u_int16_t

Please see details below. Successfully installed updated gem on:

* Win7 32bit mingw.org gcc 4.6.2 ruby 1.9.3p374 (2013-01-15 revision 38833) [i386-mingw32]
* Win7 32bit mingw-w64 gcc 4.7.2 ruby 2.0.0dev (2013-01-30 trunk 38993) [i386-mingw32]
* Arch Linux 3.7.4 gcc 4.7.2 ruby 2.0.0dev (2013-01-30 trunk 38993) [i686-linux]