powerpc: Add a prototype for mcount() so it can be versioned

Currently we get a warning that _mcount() can't be versioned:

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

Add a prototype to asm-prototypes.h to fix it.

The prototype is not really correct, mcount() is not a normal function,
it has a special ABI. But for the purpose of versioning it doesn't
matter.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/asm-prototypes.h

@@ -120,4 +120,6 @@ extern s64 __ashrdi3(s64, int);
 extern int __cmpdi2(s64, s64);
 extern int __ucmpdi2(u64, u64);
 
+void _mcount(void);
+
 #endif /* _ASM_POWERPC_ASM_PROTOTYPES_H */