sysutils/testdisk: fix build with NTFS option

A benign type mismatch in a function pointer only used when compiled
with the NTFS option was disagreeable to LLVM 16. Patch things over
by correcting the signature.

Reported by: Ale <discipline@tiscali.it>
PR: 272982
MFH: 2023Q3

(cherry picked from commit f5fac239219bdd6b380b7bad35be559d29871ef1)