make: Use -f option for rm

Otherwise I'm getting warnings (especially in zdtm)
when we are trying to remove non-generated .d files.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Makefile

@@ -13,7 +13,7 @@ endif
 FIND		:= find
 CSCOPE		:= cscope
 TAGS		:= ctags
-RM		:= rm
+RM		:= rm -f
 LD		:= ld
 CC		:= gcc
 ECHO		:= echo