include: fix arm build one more time

Fourth time's the charm.

Actually this doesn't fix the build, there is a
crash after go_bootstrap is compiled which looks
like it is related to auxv parsing.

Change-Id: Id00e2dfbe7bae42856f996065d3fb90b820e29a8
Reviewed-on: https://go-review.googlesource.com/3610
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

include/u.h

@@ -71,10 +71,6 @@ extern "C" {
 #include <ctype.h>	/* for tolower */
 #include <time.h>
 
-#ifndef PLAN9
-#include <signal.h>
-#endif
-
 /*
  * OS-specific crap
  */