Update binding.gyp to fix "napi.h not found" error

[dennisameling]

Fixes #297

For some reason, the @ in <!@(node -p \"require('node-addon-api').include\") causes node-gyp rebuild to fail with:

Building the projects in this solution one at a time. To enable parallel build, please add the "-m" switch.
  async.cc
  main.cc
  keytar_win.cc
  win_delay_load_hook.cc
C:\repos\desktop\app\node_modules\keytar\src\main.cc(1,10): fatal error C1083: Cannot open include file: 'napi.h': No such file or directory [C:\repos\desktop\app\node_modules\keytar\build\keytar.vcxproj]
C:\repos\desktop\app\node_modules\keytar\src\async.cc(4,10): fatal error C1083: Cannot open include file: 'napi.h': No such file or directory [C:\repos\desktop\app\node_modules\keytar\build\keytar.vcxproj]
gyp ERR! build error
gyp ERR! stack Error: `C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\MSBuild\Current\Bin\MSBuild.exe` failed with exit code: 1
gyp ERR! stack     at ChildProcess.onExit (C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\lib\build.js:194:23)
gyp ERR! stack     at ChildProcess.emit (events.js:314:20)
gyp ERR! stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:276:12)
gyp ERR! System Windows_NT 10.0.19042
gyp ERR! command "C:\\Program Files\\nodejs\\node.exe" "C:\\Program Files\\nodejs\\node_modules\\npm\\node_modules\\node-gyp\\bin\\node-gyp.js" "rebuild"
gyp ERR! cwd C:\repos\desktop\app\node_modules\keytar
gyp ERR! node -v v14.13.0

This PR fixes this problem by changing include_dirs to the value that is indicated in node-addon-api's official docs: https://github.com/nodejs/node-addon-api/blob/master/doc/setup.md#installation-and-usage

While #319 fixed arm64 builds on Windows by adding prebuilds for Electron (which works around the issue), it doesn't actually fix the scenario for native (re)compilation. This PR does. 🚀

[shiftkey]

Great find! Thanks for the contribution @dennisameling!