vim: move long_u from vim.h to types.h

Seems to make no difference to the main binary, but it helps the tests a bit
further along.

src/nvim/memfile_defs.h

@@ -1,6 +1,8 @@
 #ifndef NVIM_MEMFILE_DEFS_H
 #define NVIM_MEMFILE_DEFS_H
 
+#include "nvim/types.h"
+
 typedef struct block_hdr bhdr_T;
 typedef long blocknr_T;