clone: Make the 'junk_mode' symbol a file static

Sparse issues an "'junk_mode' not declared. Should it be static?"
warning. In order to suppress the warning, since this symbol does
not need more than file visibility, we simply add the static
modifier to its declaration.
Signed-off-by: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -377,7 +377,7 @@ static void clone_local(const char *src_repo, const char *dest_repo)
 static const char *junk_work_tree;
 static const char *junk_git_dir;
 static pid_t junk_pid;
-enum {
+static enum {
 	JUNK_LEAVE_NONE,
 	JUNK_LEAVE_REPO,
 	JUNK_LEAVE_ALL