hashtab.h: don't include vim.h

Including vim.h in another header filer is asking for trouble. Test code
that includes separate header files (e.g.: cimport './src/nvim/buffer.h'),
has a really bad time with this. This is just one piece of the puzzle
though.

src/nvim/hashtab.h

@@ -1,7 +1,9 @@
 #ifndef NVIM_HASHTAB_H
 #define NVIM_HASHTAB_H
 
-#include "nvim/vim.h"
+#include <stddef.h>
+
+#include "nvim/types.h"
 
 /// Type for hash number (hash calculation result).
 typedef size_t hash_T;