pgsql: oauth: Explicitly depend on -pthread

oauth: Explicitly depend on -pthread

Followup to 4e1e41733 and 52ecd05ae. oauth-utils.c uses
pthread_sigmask(), requiring -pthread on Debian bullseye at minimum.

Reported-by: Christoph Berg <myon@debian.org>
Tested-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/aK4PZgC0wuwQ5xSK%40msg.df7cb.de
Backpatch-through: 18

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/85b380162cd6c66752d1dd020a2d9700da0903c9

Modified Files
--------------
meson.build                         | 1 +
src/interfaces/libpq-oauth/Makefile | 3 ++-
2 files changed, 3 insertions(+), 1 deletion(-)