Merge #10432: [Trivial] Add BITCOIN_FS_H endif footer in fs.h

Summary:
2300a5e [Trivial] Add BITCOIN_FS_H endif footer in fs.h (Ahmad Kazi)

Tree-SHA512: fd56c5c78c12db74baeef93186b59814ac626beeecfad5db852ab8eaad8e71f7f47d64590bcdd9052cc9e97f4531ba90c1f66f0f9e6517d2f956d622803111df

Backport of Core PR10432
https://github.com/bitcoin/bitcoin/pull/10432/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, markblundeberg, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3407