function_graph: Place ftrace_graph_entry_stub() prototype in include/linux/ftrace.h

ftrace_graph_entry_stub() is defined in generic code, its prototype should
be in the generic header and not defined throughout architecture specific
code in order to use it.

Cc: Greentime Hu <green.hu@gmail.com>
Cc: Vincent Chen <deanbo422@gmail.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-parisc@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

arch/parisc/kernel/ftrace.c

@@ -51,7 +51,6 @@ void notrace __hot ftrace_function_trampoline(unsigned long parent,
 				unsigned long org_sp_gr3)
 {
 	extern ftrace_func_t ftrace_trace_function;  /* depends on CONFIG_DYNAMIC_FTRACE */
-	extern int ftrace_graph_entry_stub(struct ftrace_graph_ent *trace);
 
 	if (ftrace_trace_function != ftrace_stub) {
 		/* struct ftrace_ops *op, struct pt_regs *regs); */