riscv: signal: fix missing prototype warning

Fix the warning:
arch/riscv/kernel/signal.c:316:27: warning: no previous prototype for function 'do_notify_resume' [-Wmissing-prototypes]
asmlinkage __visible void do_notify_resume(struct pt_regs *regs,

All other functions in the file are static & none of the existing
headers stood out as an obvious location. Create signal.h to hold the
declaration.

Fixes: e2c0cdfb ("RISC-V: User-facing API")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220814141237.493457-4-mail@conchuod.ieSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/signal.h

+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef __ASM_SIGNAL_H
+#define __ASM_SIGNAL_H
+
+#include <uapi/asm/signal.h>
+#include <uapi/asm/ptrace.h>
+
+asmlinkage __visible
+void do_notify_resume(struct pt_regs *regs, unsigned long thread_info_flags);
+
+#endif

arch/riscv/kernel/signal.c

@@ -15,6 +15,7 @@
 
 #include <asm/ucontext.h>
 #include <asm/vdso.h>
+#include <asm/signal.h>
 #include <asm/signal32.h>
 #include <asm/switch_to.h>
 #include <asm/csr.h>