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"
 

src/nvim/memory.h

@@ -1,8 +1,7 @@
 #ifndef NVIM_MEMORY_H
 #define NVIM_MEMORY_H
 
-#include <stddef.h>
-#include "nvim/vim.h"
+#include <stddef.h>  // for size_t
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "memory.h.generated.h"