sparc64: fix sparse warning in tsb.c

Fix following warning:
tsb.c:290:5: warning: symbol 'sysctl_tsb_ratio' was not declared. Should it be static?

Add extern declaration in asm/setup.h and remove local declaration
in kernel/sysctl.c

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