Clean libhw subdirs as well

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/Makefile.hw b/Makefile.hw
index be35359..b9181ab 100644
--- a/Makefile.hw
+++ b/Makefile.hw
@@ -18,7 +18,7 @@
 	@true
 
 clean:
-	rm -f *.o *.d *.a *~
+	rm -f *.o */*.o *.d */*.d *.a */*.a *~ */*~
 
 # Include automatically generated dependency files
 -include $(wildcard *.d */*.d)