pgsql: Move be-gssapi-common.h into src/include/libpq/

Move be-gssapi-common.h into src/include/libpq/

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/35b2d4bc0eb5d61a2a294ccb6b2e4abdad307604

Modified Files
--------------
src/backend/libpq/auth.c | 2 +-
src/backend/libpq/be-gssapi-common.c | 2 +-
src/backend/libpq/be-secure-gssapi.c | 2 +-
src/{backend => include}/libpq/be-gssapi-common.h | 0
4 files changed, 3 insertions(+), 3 deletions(-)