patch 9.1.0959: Coverity complains about type conversion

Problem:  Coverity complains about type conversion
          (after v9.1.0957)
Solution: use size_t instead of int for file length
          (Yegappan Lakshmanan)

closes: #16297

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/proto/filepath.pro b/src/proto/filepath.pro
index 46f51cb..2979b73 100644
--- a/src/proto/filepath.pro
+++ b/src/proto/filepath.pro
@@ -1,5 +1,5 @@
 /* filepath.c */
-int modify_fname(char_u *src, int tilde_file, size_t *usedlen, char_u **fnamep, char_u **bufp, int *fnamelen);
+int modify_fname(char_u *src, int tilde_file, size_t *usedlen, char_u **fnamep, char_u **bufp, size_t *fnamelen);
 void shorten_dir(char_u *str);
 int file_is_readable(char_u *fname);
 void f_chdir(typval_T *argvars, typval_T *rettv);