scripts/Makefile.clean: clean also $(extra-m) and $(extra-)

This commit is a backport from Linux Kernel,
commit 9d5db8949f1ecf4019785b04d8986835d3c0e99e,
written by me.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

scripts/Makefile.clean

@@ -45,8 +45,8 @@ subdir-ymn	:= $(foreach f, $(subdir-ymn), \
 # build a list of files to remove, usually relative to the current
 # directory
 
-__clean-files	:= $(extra-y) $(always)                  \
-		   $(targets) $(clean-files)             \
+__clean-files	:= $(extra-y) $(extra-m) $(extra-)       \
+		   $(always) $(targets) $(clean-files)   \
 		   $(host-progs)                         \
 		   $(hostprogs-y) $(hostprogs-m) $(hostprogs-)