[RS6000] Bootstrap failure compiling xcoffout.c

git commit 41f70262f (svn rev 264868) exposed a signed/unsigned
comparison.  Fixed by matching the type of the local var to that of
the tree field.

	* xcoffout.c (do_block): Signed/unsigned warning fix.

From-SVN: r266555

gcc/ChangeLog

+2018-11-28  Alan Modra  <amodra@gmail.com>
+
+	* xcoffout.c (do_block): Signed/unsigned warning fix.
+
 2018-11-28  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/79351

gcc/xcoffout.c

@@ -345,7 +345,7 @@ xcoffout_source_line (unsigned int line, unsigned int column ATTRIBUTE_UNUSED,
    This function works by walking the tree structure of blocks,
    counting blocks until it finds the desired block.  */
 
-static int do_block = 0;
+static unsigned int do_block = 0;
 
 static void
 xcoffout_block (tree block, int depth, tree args)