sh: Fix compiler error and include the definition of IS_ERR_VALUE

sh: Fix compiler error and include the definition of IS_ERR_VALUE

When arch/sh/include/asm/syscall_32.h is included from a file that
doesn't also include linux/err.h the following error is produced,

In file included from /home/matt/src/kernels/sh-2.6/arch/sh/include/asm/syscall.h:5,
                 from kernel/trace/trace_syscalls.c:3:
/home/matt/src/kernels/sh-2.6/arch/sh/include/asm/syscall_32.h: In function 'syscall_get_error':
/home/matt/src/kernels/sh-2.6/arch/sh/include/asm/syscall_32.h:28: error: implicit declaration of function 'IS_ERR_VALUE'
make[2]: *** [kernel/trace/trace_syscalls.o] Error 1
make[1]: *** [kernel/trace] Error 2
make: *** [kernel] Error 2

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>