Fix normal build

[ArturKnopik]

Pull Request Prelude

• I have followed [proper The Forgotten Server code styling][code].
• I have read and understood the [contribution guidelines][cont] before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Fix normal build by Visual Studio.
Explicitly specifying src folder as include folder, fixing include otpch.h in http module.

Issues addressed:

Can't build project using normal build.

How to test:
Visual Studio

1. Open projec property page
2. Go to C/C++ -> Unity Build
3. Set "Include In Unity File" to no
4. Build project

Linux

1. Open CMakeList.txt in src folder
2. Remove "set_target_properties(tfslib PROPERTIES UNITY_BUILD ${ENABLE_UNITY_BUILD})"
3. Open CMakeList.txt in src/http folder
4. Remove "set_target_properties(tfslib PROPERTIES UNITY_BUILD ${ENABLE_UNITY_BUILD})"
5. Build project