resolv: Make _res_opcodes a compat symbol [BZ #31764]

_res_opcodes was exported by accident as a variable.  Fix BZ #31764 by
making _res_opcodes a compat symbol.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sunil K Pandey <skpgkp2@gmail.com>