Install missing pgxc header files properly.
Install missing pgxc header files properly.

Looks like we messed up this during the merge process. Per report from David
Rowley

oberstet/postgres-xl,yazun/postgres-xl,pavanvd/postgres-xl,oberstet/postgres-xl,zeroae/postgres-xl,yazun/postgres-xl,pavanvd/postgres-xl,pavanvd/postgres-xl,yazun/postgres-xl,zeroae/postgres-xl,pavanvd/postgres-xl,zeroae/postgres-xl,yazun/postgres-xl,zeroae/postgres-xl,oberstet/postgres-xl,pavanvd/postgres-xl,oberstet/postgres-xl,yazun/postgres-xl,zeroae/postgres-xl,oberstet/postgres-xl