memory.h: don't include vim.h in header files

Also include stdint.h in khash.h. It was transitively included by vim.h via
memory.h before. khash.h accidentally relied on that.

src/nvim/lib/khash.h

@@ -128,6 +128,7 @@ int main() {
 #include <stdlib.h>
 #include <string.h>
 #include <limits.h>
+#include <stdint.h>
 
 #include "nvim/memory.h"