ARM: 7548/1: include linux/sched.h in syscall.h

The syscall tracing patch introduces a compile bug in lttng-modules
when the latter calls syscall_get_nr(), similar to the following:

<path-to-linux>/arch/arm/include/asm/syscall.h:21:2: error: implicit declaration of function 'task_thread_info' [-Werror=implicit-function-declaration]

The issue is that we are using task_thread_info() in the
syscall_get_nr() function in asm/syscall.h, but not explicitly
including sched.h from this file, so we can expect this bug might
surface any time that syscall_get_nr() is called.

Explicitly including sched.h solves the problem.

Cc: <stable@vger.kernel.org> [3.5, 3.6]
Signed-off-by: Wade Farnsworth <wade_farnsworth@mentor.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>