Silence dpkg usr-merge warnings on PureOS.

Closes: #2

dpkg prints out a warning on systems that use usr-merge.
This warning is scary for some users.

debian/dpkg.postinst

@@ -37,6 +37,10 @@ check_merged_usr_via_aliased_dirs()
     # Ubuntu does not seem interested in it.
     return
     ;;
+  pureos)
+    # Pureos does not seem interested in it.
+    return
+    ;;  
   esac
 
   for d in /bin /sbin /lib /lib32 /libo32 /libx32 /lib64; do