MIPS: Include asm/ptrace.h now linux/sched.h doesn't

Use of the task_pt_regs() based macros in MIPS' asm/processor.h for
accessing the user context on the kernel stack need the definition of
struct pt_regs from asm/ptrace.h. __own_fpu() in asm/fpu.h uses these
macros but implicitly depended on linux/sched.h to include asm/ptrace.h.

Since commit f780d89a ("sched/headers: Remove <asm/ptrace.h> from
<linux/sched.h>") however linux/sched.h no longer includes asm/ptrace.h,
so include it explicitly from asm/fpu.h where it is needed instead.

This fixes build errors such as:

./arch/mips/include/asm/fpu.h: In function '__own_fpu':
./arch/mips/include/asm/processor.h:385:31: error: invalid application of 'sizeof' to incomplete type 'struct pt_regs'
     THREAD_SIZE - 32 - sizeof(struct pt_regs))
                               ^

Fixes: f780d89a ("sched/headers: Remove <asm/ptrace.h> from <linux/sched.h>")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15386/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/fpu.h

@@ -20,6 +20,7 @@
 #include <asm/cpu-features.h>
 #include <asm/fpu_emulator.h>
 #include <asm/hazards.h>
+#include <asm/ptrace.h>
 #include <asm/processor.h>
 #include <asm/current.h>
 #include <asm/msa.h>