asm-generic: unistd.h: Wire up sys_rseq

The new rseq call arrived in 4.18-rc1, so provide it in the asm-generic
unistd.h for architectures such as arm64.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>