walletdb: Make SpliWalletFilePath non-static

Summary:
Partial backport (1/5) of core PR19290:
https://github.com/bitcoin/bitcoin/pull/19290/commits/c3538f435af8c408759d9d005e80b2f1690e0659

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8611