No werror=true

MSVC throw a lot of warning because we also must re-export stl symbols
used in our exported class.

See [1] and [2] (and links inside) for more information.

Especially [3] which seems to say that we should have issue at link time if
it compatibility issue occurs (and so easily catchable).

So I just remove the `werror=true` for now.

[1] https://stackoverflow.com/questions/16419318/one-way-of-eliminating-c4251-warning-when-using-stl-classes-in-the-dll-interface
[2] https://stackoverflow.com/questions/2132747/warning-c4251-when-building-a-dll-that-exports-a-class-containing-an-atlcstrin
[3] https://stackoverflow.com/questions/2132747/warning-c4251-when-building-a-dll-that-exports-a-class-containing-an-atlcstrin#comment66255284_4563701

.github/workflows/ci.yml

@@ -131,7 +131,7 @@ jobs:
         run: |
           set PKG_CONFIG_PATH=%cd%\BUILD_win-amd64\INSTALL\lib\pkgconfig
           dir %PKG_CONFIG_PATH%
-          meson.exe setup . build -Dwith_xapian=false
+          meson.exe setup . build -Dwith_xapian=false -Dwerror=false
           cd build
           ninja.exe