[SPARC64]: Kconfig fix (GEN_RTC dependencies)

Yet another architecture not coverd by GEN_RTC - sparc64 never picked
it until now and it doesn't have asm/rtc.h to go with it, so it
wouldn't compile anyway (or have these ioctls in the user-visible
headers, for that matter).

FWIW, I'm very tempted to introduce ARCH_HAS_GEN_RTC and have it set
in arch/*/Kconfig for architectures that know what to do with this
stuff - for something supposedly generic the list of architectures
where it doesn't work is getting too long...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>