reftable/system: move "dir.h" to its only user

We still include "dir.h" in "reftable/system.h" even though it is not
used by anything but by a single unit test. Move it over into that unit
test so that we don't accidentally use any functionality provided by it
in the reftable codebase.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/system.h

@@ -15,7 +15,6 @@ https://developers.google.com/open-source/licenses/bsd
 #include "lockfile.h"
 #include "tempfile.h"
 #include "hash.h" /* hash ID, sizes.*/
-#include "dir.h" /* remove_dir_recursively, for tests.*/
 
 int hash_size(uint32_t id);
 

t/unit-tests/t-reftable-stack.c

@@ -8,6 +8,7 @@ https://developers.google.com/open-source/licenses/bsd
 
 #include "test-lib.h"
 #include "lib-reftable.h"
+#include "dir.h"
 #include "reftable/merged.h"
 #include "reftable/reader.h"
 #include "reftable/reftable-error.h"