fix compile warning in pg_id

I get the following warning when I compile src/bin/pg_id/pg_id.c:

gcc -pipe -g -Wall -Wmissing-prototypes -Wmissing-declarations -I../../../src/include   -c -o pg_id.o pg_id.c -MMD
pg_id.c: In function `main':
pg_id.c:35: warning: unused variable `optarg'

The attached trivial patch fixes the warning by removing the variable.

-Neil