um: start switching the references to host mcontext_t to its userland type

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/arch/um/os-Linux/internal.h b/arch/um/os-Linux/internal.h
new file mode 100644
index 0000000..2c3c3ec
--- /dev/null
+++ b/arch/um/os-Linux/internal.h
@@ -0,0 +1 @@
+void alarm_handler(int, mcontext_t *);