StdLib/sys/termios: Define cc_t as unsigned

StdLib/sys/termios: Define cc_t as unsigned

According to the POSIX standard, cc_t, speed_t, and tcflag_t should be
unsigned integer types. Define cc_t as unsigned to match POSIX and fix
an implicit conversion error when building StdLib with XCODE5/CLANG38.

Cc: Daryl McDaniel <edk2-lists@mc2research.org>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Alex James <theracermaster@gmail.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>