upas: make 'all' the default target

When consolidating the duplicated targets, the
one that actually got built ended up arbitrary.
Put in a 'default' target that runs 'mk all'.

sys/src/cmd/upas/mkfile

@@ -24,6 +24,9 @@ PROGS=\
 #libs must be made first
 DIRS=$LIBS $PROGS
 
+default:V:
+	mk all
+
 all install installall clean nuke:V:
 	for (i in $DIRS) @{
 		cd $i