refactor: enable -Wconversion warning for screen.c

Closes https://github.com/neovim/neovim/issues/567

src/nvim/fold.h

@@ -18,7 +18,7 @@ typedef struct foldinfo {
                                 // other fields are invalid
   int fi_low_level;             // lowest fold level that starts in the same
                                 // line
-  long fi_lines;
+  linenr_T fi_lines;
 } foldinfo_T;
 
 #define FOLDINFO_INIT { 0, 0, 0, 0 }