spec: Move timer interrupt unmasking option

This moves the timer interrupt unmasking configuration option to make it
more available to other BSPs that might need it.

spec/build/bsps/arm/xen/bspxen.yml

@@ -29,7 +29,7 @@ links:
 - role: build-dependency
   uid: ../optgtusevirt
 - role: build-dependency
-  uid: optgentmunmask
+  uid: ../../optgentmunmask
 - role: build-dependency
   uid: optloadoff
 - role: build-dependency