Merge #11495: [trivial] Make namespace explicit for is_regular_file

Summary:
f4c4e38 [trivial] Make namespace explicit for is_regular_file (John Newbery)

Pull request description:

is_regular_file resolves using argument dependent lookup. Make the
namespace explicit so it's obvious where the function is defined.

For those not familiar with argument dependent lookups:

- http://en.cppreference.com/w/cpp/language/adl
- https://en.wikipedia.org/wiki/Argument-dependent_name_lookup

Thanks to C++ guru @ryanofsky for pointing this out to me.

Tree-SHA512: 919f1818081a8f90c5751181f87e13b06d90f8aec0ab873100434e55c85cca6e0e288ecc7f135e19e9b5dba7952e96b6393864b7840e20b69dd40e92a157928b

Backport of Core PR11495
https://github.com/bitcoin/bitcoin/pull/11495/

Test Plan:

make check

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

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3685