x86, um: sanitize uml sigcontext.h uses

a) the only difference between sigcontext and sysdep/sigcontext
is that the former contains externs for two long-dead functions.
Removed, switched the only user to sysdep/sigcontext

b) asm/sigcontext.h is removable - that of underlying architecture
would get used.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/um/kernel/signal.c

@@ -11,7 +11,7 @@
 #include <asm/unistd.h>
 #include "frame_kern.h"
 #include "kern_util.h"
-#include "sigcontext.h"
+#include <sysdep/sigcontext.h>
 
 EXPORT_SYMBOL(block_signals);
 EXPORT_SYMBOL(unblock_signals);