pgsql: Prevent compilation of frontend-only files in src/common/ with b

Prevent compilation of frontend-only files in src/common/ with backend

Any frontend-only file of src/common/ should include a protection to
prevent such code to be included in the backend compilation.
fe_memutils.c and restricted_token.c have been doing that, while
file_utils.c (since bf5bb2e) and logging.c (since fc9a62a) forgot it.

Reviewed-by: Daniel Gustafsson
Discussion: /messages/by-id/20200625080757.GI130132@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/324435eb14e4f41cd430f96c9b13ad9b160e45e4

Modified Files
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src/common/file_utils.c | 5 +++++
src/common/logging.c | 5 +++++
2 files changed, 10 insertions(+)