I just tested myself and also works here, so I guess it is something specific to your environment. Can you try with a fresh clone of kamailio?
``` kamailio-dev$ make modules modules=src/modules/db_postgres make -C src/ modules modules=modules/db_postgres make[1]: Entering directory '/home/milano/work/sip/esr/kamailio-dev/src' CC (gcc) [M db_postgres.so] km_dbase.o CC (gcc) [M db_postgres.so] km_val.o CC (gcc) [M db_postgres.so] pg_oid.o CC (gcc) [M db_postgres.so] pg_fld.o CC (gcc) [M db_postgres.so] pg_con.o CC (gcc) [M db_postgres.so] pg_mod.o CC (gcc) [M db_postgres.so] km_res.o CC (gcc) [M db_postgres.so] pg_uri.o CC (gcc) [M db_postgres.so] pg_sql.o CC (gcc) [M db_postgres.so] pg_res.o CC (gcc) [M db_postgres.so] km_pg_con.o CC (gcc) [M db_postgres.so] db_postgres.o CC (gcc) [M db_postgres.so] pg_cmd.o CC (gcc) [L libsrdb2.so.1.0] db_fld.o CC (gcc) [L libsrdb2.so.1.0] db_con.o CC (gcc) [L libsrdb2.so.1.0] db_res.o CC (gcc) [L libsrdb2.so.1.0] db_rec.o CC (gcc) [L libsrdb2.so.1.0] db_drv.o CC (gcc) [L libsrdb2.so.1.0] db_gen.o CC (gcc) [L libsrdb2.so.1.0] db_pool.o CC (gcc) [L libsrdb2.so.1.0] db_cmd.o CC (gcc) [L libsrdb2.so.1.0] db.o CC (gcc) [L libsrdb2.so.1.0] db_uri.o CC (gcc) [L libsrdb2.so.1.0] db_ctx.o LD (gcc) [L libsrdb2.so.1.0] libsrdb2.so.1.0 CC (gcc) [L libsrdb1.so.1.0] db_query.o CC (gcc) [L libsrdb1.so.1.0] db_id.o CC (gcc) [L libsrdb1.so.1.0] db.o CC (gcc) [L libsrdb1.so.1.0] db_row.o CC (gcc) [L libsrdb1.so.1.0] db_res.o CC (gcc) [L libsrdb1.so.1.0] db_pool.o CC (gcc) [L libsrdb1.so.1.0] db_ut.o CC (gcc) [L libsrdb1.so.1.0] db_val.o LD (gcc) [L libsrdb1.so.1.0] libsrdb1.so.1.0 LD (gcc) [M db_postgres.so] db_postgres.so make[1]: Leaving directory '/home/milano/work/sip/esr/kamailio-dev/src' ```