Don't include postgres.h in postgres_fe.h for cpluspluscheck.

Error exposed by recent Assert changes.

Complaint from Peter Eisentraut.

src/tools/pginclude/cpluspluscheck

@@ -25,7 +25,7 @@ for f in `find src/include src/interfaces/libpq/libpq-fe.h src/interfaces/libpq/
 do
 	{
 	    echo ' extern "C" {'
-	    echo '#include "postgres.h"'
+	    test $f != "src/include/postgres_fe.h" && echo '#include "postgres.h"'
 	    echo "#include \"$f\""
 	    echo '};'
 	} >$tmp/test.cpp