nsis installer: Suppress "ANSI targets are deprecated" warning

nsis installer: Suppress "ANSI targets are deprecated" warning

When we build our Windows installer, it emits the warning:

   warning 7998: ANSI targets are deprecated

Fix this by making our installer a Unicode installer instead.  These
won't work on Win95/98/ME, but we already do not support those.

See
https://nsis.sourceforge.io/Docs/Chapter4.html#aunicodetarget
for the documentation of the Unicode directive.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-id: 20220305105743.2384766-3-peter.maydell@linaro.org