xtensa: sysdep.h needs to include sys/syscall.h

The Xtensa version of sysdep.h didn't include sys/syscall.h. This header
file is, for example, needed by not-cancel.h, which uses the
INTERNAL_SYSCALL macro.

Signed-off-by: Chris Zankel <chris@zankel.net>

libc/sysdeps/linux/xtensa/sysdep.h

@@ -17,6 +17,8 @@
    Software Foundation, Inc., 51 Franklin Street - Fifth Floor,
    Boston, MA 02110-1301, USA.  */
 
+#include <sys/syscall.h>
+
 #ifdef __ASSEMBLER__
 
 #define ALIGNARG(log2) 1 << log2