environment.h: move declarations for environment.c functions from cache.h

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/worktree.c b/builtin/worktree.c
index 1533b4a..d9345ef 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -4,6 +4,7 @@
 #include "config.h"
 #include "builtin.h"
 #include "dir.h"
+#include "environment.h"
 #include "gettext.h"
 #include "hex.h"
 #include "parse-options.h"