mark gmp-xen and minios-xen as unmaintained

these are relicts from earlier MirageOS compilation strategies, and have not
been used since mirage 3

Author: hannesm

packages/gmp-xen/gmp-xen.6.0.0-1/opam

@@ -58,3 +58,4 @@ extra-source "gmp-6a.diff" {
     "md5=ef379d00b52fd8138627e8d13606736c"
   ]
 }
+x-maintenance-intent: [ "(none)" ]

packages/minios-xen/minios-xen.0.9/opam

@@ -44,3 +44,4 @@ extra-source "build-opam.sh" {
     "md5=93746a0c93f75851c8a8f81e7e8b5d82"
   ]
 }
+x-maintenance-intent: [ "(none)" ]