Makefile: fixup dependency problem for files in sub dirs

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/.gitignore b/.gitignore
index 131f9a9..2457d65 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 fio
 *.o
+*.d
 .depend
 cscope.out