pgsql: Call genhtml with --prefix option so local build paths don't

Log Message:
-----------
Call genhtml with --prefix option so local build paths don't appear in the
coverage output.

Modified Files:
--------------
pgsql:
GNUmakefile.in (r1.48 -> r1.49)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/GNUmakefile.in?r1=1.48&r2=1.49)