refactor: the long goodbye

long is 32 bits on windows, while it is 64 bits on other architectures.
This makes the type suboptimal for a codebase meant to be
cross-platform. Replace it with more appropriate integer types.

src/nvim/ex_getln.c

@@ -2208,7 +2208,7 @@ handle_T cmdpreview_get_bufnr(void)
   return cmdpreview_bufnr;
 }
 
-long cmdpreview_get_ns(void)
+int cmdpreview_get_ns(void)
 {
   return cmdpreview_ns;
 }