[libwebsockets] Fix usage, export include path

[JackBoosY]

Both the marco LIBWEBSOCKETS_INCLUDE_DIR and the INTERFACE_INLUCDE_DIR is empty:

CMake Error in CMakeLists.txt:
  Found relative path while evaluating include directories of "ZERO_CHECK":

    "LIBWEBSOCKETS_INCLUDE_DIR-NOTFOUND

set_target_properties(websockets_shared PROPERTIES
  INTERFACE_LINK_LIBRARIES "${_IMPORT_PREFIX}/lib/libssl.lib;${_IMPORT_PREFIX}/lib/libcrypto.lib;\$<\$<NOT:\$<CONFIG:DEBUG>>:${_IMPORT_PREFIX}/lib/pthreadVC3.lib>;\$<\$<CONFIG:DEBUG>:${_IMPORT_PREFIX}/debug/lib/pthreadVC3d.lib>;\$<\$<NOT:\$<CONFIG:DEBUG>>:${_IMPORT_PREFIX}/lib/pthreadVC3.lib>;\$<\$<CONFIG:DEBUG>:${_IMPORT_PREFIX}/debug/lib/pthreadVC3d.lib>;ws2_32.lib;userenv.lib;psapi.lib;iphlpapi.lib;crypt32.lib;\$<\$<NOT:\$<CONFIG:DEBUG>>:${_IMPORT_PREFIX}/lib/zlib.lib>;\$<\$<CONFIG:DEBUG>:${_IMPORT_PREFIX}/debug/lib/zlibd.lib>;${_IMPORT_PREFIX}/lib/libuv.lib"
)

Fix this.

Fixes #16466.

[JackBoosY]

@kzhdev Can you please test this PR?

Thanks.

[JackBoosY]

Depend on #17011.

[JackBoosY]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[kzhdev]

@JackBoosY Verified the issue is fixed. Thanks.