fdo#65124 define HAVE_FMERGENEUTRALITEMS for MSVC build

It has been added because mingw headers are missing
fMergeNeutralItems in SCRIPT_CONTROL struct
(see commit f6deda85ef4f28426c09fab7205f274922af3d96),
so there is no apparent reason to disable it for MSVC.
It also resolves fdo#65124.

Change-Id: I4b7381f076c213f12f32cf03e2c0f321ccb1c4ce
Reviewed-on: https://gerrit.libreoffice.org/6711
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>