tar: move FNM_LEADING_DIR #define to tar.c - no other users.
Closes bug 1332.

diff --git a/archival/tar.c b/archival/tar.c
index 64975c4..6b37383 100644
--- a/archival/tar.c
+++ b/archival/tar.c
@@ -28,6 +28,11 @@
 #include "libbb.h"
 #include "unarchive.h"
 
+/* FIXME: Stop using this non-standard feature */
+#ifndef FNM_LEADING_DIR
+#define FNM_LEADING_DIR 0
+#endif
+
 #define block_buf bb_common_bufsiz1
 
 #if ENABLE_FEATURE_TAR_CREATE