patch 9.0.1336: functions without arguments are not always declared properly

Problem:    Functions without arguments are not always declared properly.
Solution:   Use "(void)" instead of "()". (Yegappan Lakshmanan, closes #12031)
diff --git a/src/proto/filepath.pro b/src/proto/filepath.pro
index 2970e50..fd8de80 100644
--- a/src/proto/filepath.pro
+++ b/src/proto/filepath.pro
@@ -1,11 +1,11 @@
 /* filepath.c */
 int modify_fname(char_u *src, int tilde_file, int *usedlen, char_u **fnamep, char_u **bufp, int *fnamelen);
 void shorten_dir(char_u *str);
+int file_is_readable(char_u *fname);
 void f_chdir(typval_T *argvars, typval_T *rettv);
 void f_delete(typval_T *argvars, typval_T *rettv);
 void f_executable(typval_T *argvars, typval_T *rettv);
 void f_exepath(typval_T *argvars, typval_T *rettv);
-int file_is_readable(char_u *fname);
 void f_filereadable(typval_T *argvars, typval_T *rettv);
 void f_filewritable(typval_T *argvars, typval_T *rettv);
 void f_finddir(typval_T *argvars, typval_T *rettv);