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>

StdLib/Include/sys/termios.h

@@ -152,7 +152,7 @@ typedef enum {
 #define NOFLSH    0x0800    /* don't flush output on signal */
 #define FLUSHO    0x1000    /* output being flushed (state) */
 
-typedef INT8    cc_t;
+typedef UINT8   cc_t;
 typedef UINT16  tcflag_t;
 typedef UINT32  speed_t;