fix(build): export symbols on Windows

Closes https://github.com/neovim/neovim/issues/15063

Allows using Neovim core functions using LuaJIT FFI on Windows.

test/functional/lua/ffi_spec.lua

@@ -0,0 +1,62 @@
+local helpers = require('test.functional.helpers')(after_each)
+local eq = helpers.eq
+local exec_lua = helpers.exec_lua
+local clear = helpers.clear
+
+before_each(clear)
+
+describe('ffi.cdef', function()
+  it('can use Neovim core functions', function()
+    if not exec_lua("return pcall(require, 'ffi')") then
+      pending('missing LuaJIT FFI')
+    end
+
+    eq(12, exec_lua[[
+      local ffi = require('ffi')
+
+      ffi.cdef('int curwin_col_off(void);')
+
+      vim.cmd('set number numberwidth=4 signcolumn=yes:4')
+
+      return ffi.C.curwin_col_off()
+    ]])
+
+    eq(20, exec_lua[=[
+      local ffi = require('ffi')
+
+      ffi.cdef[[
+        typedef unsigned char char_u;
+        typedef struct window_S win_T;
+        typedef struct {} stl_hlrec_t;
+        typedef struct {} StlClickRecord;
+        typedef struct {} Error;
+
+        win_T *find_window_by_handle(int Window, Error *err);
+
+        int build_stl_str_hl(
+          win_T *wp,
+          char_u *out,
+          size_t outlen,
+          char_u *fmt,
+          int use_sandbox,
+          char_u fillchar,
+          int maxwidth,
+          stl_hlrec_t **hltab,
+          StlClickRecord **tabtab
+        );
+      ]]
+
+      return ffi.C.build_stl_str_hl(
+        ffi.C.find_window_by_handle(0, ffi.new('Error')),
+        ffi.new('char_u[1024]'),
+        1024,
+        ffi.cast('char_u*', 'StatusLineOfLength20'),
+        0,
+        0,
+        0,
+        nil,
+        nil
+      )
+    ]=])
+  end)
+end)