pgsql: Minor #include cleanup.

Log Message:
-----------
Minor #include cleanup.

I just noticed that libpq's pqsignal.h was violating our general inclusion
style guidelines by explicitly including postgres_fe.h.  Remove that, and
put it in pqsignal.c where it belongs.

Modified Files:
--------------
    pgsql/src/interfaces/libpq:
        pqsignal.c (r1.30 -> r1.31)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/pqsignal.c?r1=1.30&r2=1.31)
        pqsignal.h (r1.25 -> r1.26)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/pqsignal.h?r1=1.25&r2=1.26)