qt: Makefile.am: symlink vlc-qt-check to modules/

vlc-qt-check is found using VLC_PKG_LIBEXEC_DIR which falls back to VLC_LIB_PATH. In vlc-static, VLC_LIB_PATH is defined by the following snippet:

#ifdef TOP_BUILDDIR
    setenv ("VLC_PLUGIN_PATH", TOP_BUILDDIR"/modules", 1);
    setenv ("VLC_DATA_PATH", TOP_SRCDIR"/share", 1);
    setenv ("VLC_LIB_PATH", TOP_BUILDDIR"/modules", 1);
#endif

With the previous changes in ddfc6d0d, the vlc-qt-check executable is not built into TOP_BUILDDIR/modules/ anymore, but in TOP_BUILDDIR/modules/gui/qt/ instead, and won't be found on platforms requiring it.

When installing, it is still located at the right place, but this commit adds a symbolic link to modules/ to ensure we can also find it with vlc-static.