ecpg: fix amd64 warning/bug

I see the following when compiling current sources on Linux/AMD64, gcc
4.0.2:

% make -s
[...]src/interfaces/ecpg/ecpglib/descriptor.c: In function
'ECPGset_desc':
[...]src/interfaces/ecpg/ecpglib/descriptor.c:509: warning: passing
argument 1 of 'ECPGget_variable' from incompatible pointer type

(Long paths shortened for brevity.) This seems to result from the patch
by Christof Petig that changes the signature of ECPGget_variable() on
certain platforms, "to work around gcc bug on powerpc and amd64."

http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/ecpglib/execute.c.diff?r1=1.38;r2=1.38.4.1
http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/ecpglib/extern.h.diff?r1=1.11;r2=1.11.4.1

Christof's patch updated execute.c and extern.h, but neglected to update
descriptor.c, resulting in the above warning (and I would imagine a
crash at runtime if this code is executed on AMD64/PPC, although I
haven't checked).

Attached is a patch that modifies the call to ECPG_get_variable() in
descriptor.c when compiled with GCC on AMD64 and PPC. Barring any
objections, I'll apply this to HEAD and the relevant stable branches
tomorrow or the day after.

-Neil