RISC-V: include linux/ftrace.h in asm-prototypes.h

Building a riscv kernel with CONFIG_FUNCTION_TRACER and
CONFIG_MODVERSIONS enabled results in these two warnings:

  MODPOST vmlinux.o
WARNING: EXPORT symbol "return_to_handler" [vmlinux] version generation failed, symbol will not be versioned.
WARNING: EXPORT symbol "_mcount" [vmlinux] version generation failed, symbol will not be versioned.

When exporting symbols from an assembly file, the MODVERSIONS code
requires their prototypes to be defined in asm-prototypes.h (see
scripts/Makefile.build). Since both of these symbols have prototypes
defined in linux/ftrace.h, include this header from RISC-V's
asm-prototypes.h.

Reported-by: Karsten Merker <merker@debian.org>
Signed-off-by: James Cowgill <jcowgill@debian.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>