sigaction: Fix !HAS_REALTIME compilation

In file included from ./include/signal.h:279:0,
				 from libc/sysdeps/linux/x86_64/sigaction.c:22:
./include/bits/sigaction.h:32:29: error: unknown type name 'siginfo_t'
   void (*sa_sigaction)(int, siginfo_t *, void *);

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>