Use unsafe block to activate the ID map

The latest release of the aarch64-paging crate marks the activate()
function as unsafe, so we must invoke it from an unsafe {} block.

Test: build tested only
Change-Id: Id729b4c0ed321ce57c22fe5125eebaf8d436568d
diff --git a/vmbase/src/memory/page_table.rs b/vmbase/src/memory/page_table.rs
index ad164b4..dc346e7 100644
--- a/vmbase/src/memory/page_table.rs
+++ b/vmbase/src/memory/page_table.rs
@@ -83,7 +83,9 @@
     /// code being currently executed. Otherwise, the Rust execution model (on which the borrow
     /// checker relies) would be violated.
     pub unsafe fn activate(&mut self) {
-        self.idmap.activate()
+        // SAFETY: the caller of this unsafe function asserts that switching to a different
+        // translation is safe
+        unsafe { self.idmap.activate() }
     }
 
     /// Maps the given range of virtual addresses to the physical addresses as lazily mapped