[shapelib] Fix shapelib usage

[JonLiu1993]

Describe the pull request

• What does your PR fix?

  Fixes [shapelib] Unable to find shp-config.cmake when including in build #16041

Shapelib unable to find shp-config.cmake when including in build

[cmake] CMake Error at external/vcpkg/scripts/buildsystems/vcpkg.cmake:650 (_find_package):
[cmake]   Could not find a package configuration file provided by "shp" with any of
[cmake]   the following names:
[cmake] 
[cmake]     shpConfig.cmake
[cmake]     shp-config.cmake
[cmake] 

Export the shpConfig file can solved the bug

I have test the usage successfully on my local machine:

PS F:\test9\test\build> F:\test9\vcpkg\downloads\tools\cmake-3.21.1-windows\cmake-3.21.1-windows-i386\bin\cmake.exe -G "Visual Studio 16 2019" -A Win32 -DCMAKE_TOOLCHAIN_FILE=F:/test9/vcpkg/scripts/buildsystems/vcpkg.cmake ..
-- Selecting Windows SDK version 10.0.19041.0 to target Windows 10.0.19042.
-- Configuring done
-- Generating done
-- Build files have been written to: F:/test9/test/build

PS F:\test9\test\build> F:\test9\vcpkg\downloads\tools\cmake-3.21.1-windows\cmake-3.21.1-windows-i386\bin\cmake.exe --build .
Microsoft (R) Build Engine version 16.11.2+f32259642 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

  main.cpp
  main.vcxproj -> F:\test9\test\build\Debug\main.exe
  Building Custom Rule F:/test9/test/CMakeLists.txt

[github-actions]

You have modified or added at least one vcpkg.json where a "license" field is missing.
If you feel able to do so, please consider adding a "license" field to the following files:

• ports/shapelib/vcpkg.json

Valid values for the license field are listed at https://spdx.org/licenses/

[JackBoosY]

Also please report this to the upstream plase.

[BillyONeal]

Thanks for the fix!