MIPS: Fix modversioning of _mcount symbol

In commit 827456e7 ("MIPS: Export _mcount alongside its definition")
the EXPORT_SYMBOL macro exporting _mcount was moved from C code into
assembly. Unlike C, exported assembly symbols need to have a function
prototype in asm/asm-prototypes.h for modversions to work properly.
Without this, modpost prints out this warning:

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

Fix by including asm/ftrace.h (where _mcount is declared) in
asm/asm-prototypes.h.

Fixes: 827456e7 ("MIPS: Export _mcount alongside its definition")
Signed-off-by: James Cowgill <James.Cowgill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15952/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>