vlccore: remove duplicated symbol

This fixes a linkage error on OS/2:

make.exe[3]: Entering directory `F:/lang/work/vlc/vlc.git/src'
  CCLD     libvlccore.la
weakld: F:/lang/work/vlc/vlc.git/src/.libs/vlccore.def - warning: Export '_vlc_latch_is_ready' (int '_vlc_latch_is_ready') is already defined.
Warning! W1121: file ldwUtpCd.: '_vlc_latch_is_ready' has already been exported
make.exe[3]: Leaving directory `F:/lang/work/vlc/vlc.git/src'

[...]

make.exe[4]: Entering directory `F:/lang/work/vlc/vlc.git/modules'
  CCLD     libvlc_http.la
  CCLD     libhttps_plugin.la
emxomf: Symbol multiply defined: _vlc_latch_is_ready
emxomfld: a.out to omf conversion failed for '../src/.libs/vlccore_dll.a'.
make.exe[4]: *** [libhttps_plugin.la] Error 1