Silence annoying warnings from GNU ar

In (very) modern versions of binutils, for instance v2.25.1 in Ubuntu
15.10, complain about VERY common `crus` argument.  This patch attempts
to silence this noise:

    $ make -j5
      CC      uev.o
      CC      io.o
      CC      timer.o
      CC      signal.o
      ARCHIVE libuev.a
      LINK    libuev.so.1
    ar: `u' modifier ignored since `D' is the default (see `U')

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Makefile

@@ -82,7 +82,7 @@ $(SOLIB): Makefile $(OBJS)
 
 $(STATICLIB): Makefile $(OBJS)
 	@printf "  ARCHIVE $@\n"
-	@$(AR) $(ARFLAGS) $@ $(OBJS)
+	@$(AR) $(ARFLAGS) $@ $(OBJS) 2>/dev/null
 
 install-exec: all
 	@printf "  INSTALL $(DESTDIR)$(libdir)/$(SOLIB)\n"