x86/ftrace: Remove mcount() declaration

Commit 562e14f7 ("ftrace/x86: Remove mcount support") removed the
support for mcount, but forgot to remove the mcount() declaration.

Clean it up.
Signed-off-by: NJisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r20190826170150.10f101ba@xhacker.debian

arch/x86/include/asm/ftrace.h

@@ -16,7 +16,6 @@
 #define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 
 #ifndef __ASSEMBLY__
-extern void mcount(void);
 extern atomic_t modifying_ftrace_code;
 extern void __fentry__(void);