[libc] Ensure that error numbers from EUNIQ() have the correct type

Error numbers are signed ints.  EUNIQ() should not allow implicit type
promotion based on the supplied error diambiguator, because this
causes problems with statements such as

  rc = ( condition ? -EUNIQ ( EBASE, disambiguator ) : -EBASE );

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/errno.h

  */
 #define EUNIQ( errno, uniq, ... ) ( {					\
 	euniq_discard ( 0, ##__VA_ARGS__);				\
-	( (errno) | ( (uniq) << 8 ) ); } )
+	( ( int ) ( (errno) | ( (uniq) << 8 ) ) ); } )
 static inline void euniq_discard ( int dummy __unused, ... ) {}
 
 /**