fix(c): fix the goto label in the HAVE_POLL branch of wait_c_impl()

fix(c): fix the goto label in the HAVE_POLL branch of wait_c_impl()

This resolves the following warning when building the extension module:

    $ python psycopg_c/setup.py build_ext -i
    [...]
    building 'psycopg_c._psycopg' extension
    x86_64-linux-gnu-gcc -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -I/home/denis/.local/pgsql/include -I/home/denis/src/psycopg3/.venv/include -I/usr/include/python3.11 -c psycopg_c/_psycopg.c -o build/temp.linux-x86_64-cpython-311/psycopg_c/_psycopg.o
    psycopg_c/_psycopg.c: In function ‘wait_c_impl’:
    psycopg_c/_psycopg.c:1649:1: warning: label ‘error’ defined but not used [-Wunused-label]
     1649 |
          | ^