sparc64: fix sparse warning in ftrace.c

Fix following warning:
ftrace.c:123:15: warning: symbol 'prepare_ftrace_return' was not declared. Should it be static?

Add prototype for asm/ftrace.h
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/include/asm/ftrace.h

@@ -22,4 +22,8 @@ struct dyn_arch_ftrace {
 };
 #endif /*  CONFIG_DYNAMIC_FTRACE */
 
+unsigned long prepare_ftrace_return(unsigned long parent,
+				    unsigned long self_addr,
+				    unsigned long frame_pointer);
+
 #endif /* _ASM_SPARC64_FTRACE */