Fixed include dir path for cmake

[WebmasterTD]

Description

When this library is used with CMake FetchContent:

FetchContent_Declare(
  wsServer
  GIT_REPOSITORY https://github.com/Theldus/wsServer.git
)
FetchContent_MakeAvailable(wsServer)

the project fails to compile due to missing include directory.
This PR fixes that.

Checklist

• I've read the notice in the PR template before submitting it
• My PR is:
  • Trivial and:
    • I haven't created an issue

[Theldus]

Hi @WebmasterTD,
Thanks for spotting this, merged.