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/exec-cmd.c b/exec-cmd.c
index 042d924..fae0d4b 100644
--- a/exec-cmd.c
+++ b/exec-cmd.c
@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "abspath.h"
+#include "environment.h"
 #include "exec-cmd.h"
 #include "gettext.h"
 #include "quote.h"