Re: Make contrib modules' installation scripts more secure.

I think this change neglected to add plpgsql to the extension
dependencies in the .control file:

12:53:51 #   Failed test 'psql -qc 'CREATE EXTENSION "cube"''
12:53:51 #   at t/TestLib.pm line 213.
12:53:51 not ok 68 - psql -qc 'CREATE EXTENSION "cube"'
12:53:51 #          got: '1'
12:53:51 #     expected: '0'
12:53:51 not ok 69 - extension cube installs without error
12:53:51 #   Failed test 'extension cube installs without error'
12:53:51 #   at t/TestLib.pm line 214.
12:53:51 #          got: 'ERROR:  language "plpgsql" does not exist
12:53:51 # HINT:  Use CREATE EXTENSION to load the language into the database.
12:53:51 # '

(The Debian regression tests remove plpgsql before testing all
extensions in turn.)

Christoph