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: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>