The file has been introduced in src/backend/libpq/ as of b0b39f72, but
all backend-side headers of libpq are located in src/include/libpq/.
Note that the identification path on top of the file referred to
src/include/libpq/ from the start.

Author: Michael Paquier
Reviewed-by: Stephen Frost
Discussion: https://postgr.es/m/20190607043415.GE1736@paquier.xyz