access_mms: fix linkage on OS/2

access_mms: fix linkage on OS/2

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  CCLD     libaccess_mms_plugin.la
weakld: error: Unresolved symbol (UNDEF) '_vlc_poll'.
weakld: info: The symbol is referenced by:
    P:\tmp\ldconv_libaccess_mms_plugin_la-mmstu_38da54703f4316fd10.obj
Ignoring unresolved externals reported from weak prelinker.
Error! E2028: _vlc_poll is an undefined reference
file P:/tmp\ldconv_libaccess_mms_plugin_la-mmstu_38da54703f4316fd10.obj(ldconv_libaccess_mms_plugin_la-mmstu_38da54703f4316fd10.obj): undefined symbol _vlc_poll
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Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>