patch: Add PGXS support to hstore's Makefile (trivial)

I discovered today that hstore's Makefile currently does not support
PGXS; namely, the ability to install it like so:

    USE_PGXS=1 make
    sudo USE_PGXS=1 make install

Moreover, hstore is the *only* contrib module whose Makefile does not
contain "PGXS".

The attached patch changes hstore's Makefile so it is like the others.
 I have tested the above commands on a copy of the hstore directory
located outside of the PostgreSQL source tree, and it appears to work
just fine.  I also tested conventional make and make install, and it
still appears to work as well.


Joey Adams