Move selinux dependencies out of header files.

Bug: http://b/27764900
Change-Id: Ib62a59edcb13054f40f514c404d32b87b14ed5f1
diff --git a/minzip/DirUtil.h b/minzip/DirUtil.h
index 85a0012..85b83c3 100644
--- a/minzip/DirUtil.h
+++ b/minzip/DirUtil.h
@@ -24,8 +24,7 @@
 extern "C" {
 #endif
 
-#include <selinux/selinux.h>
-#include <selinux/label.h>
+struct selabel_handle;
 
 /* Like "mkdir -p", try to guarantee that all directories
  * specified in path are present, creating as many directories