Hello Neil,
These function are not available in FatFs 0.09 release, however, they are included in newest FatFs version (0.11a) that is included in SDK 2.0.
You only need to define _USE_FIND as 1 instead of 0 (by default it is disabled):
#define _USE_FIND 0
/* This option switches filtered directory read feature and related functions,
/ f_findfirst() and f_findnext(). (0:Disable or 1:Enable) */
In order to have these functions (f_findnext and f_findfirst) available on ff.c file.
I hope this can help you!
Regards,
Isaac