Missing target_include_directories in libJSBSim

[carlocorradini]

I'm submitting a ...

• bug report
• feature request
• support request => Please do not submit support request here, see note at the top of this template.

Describe the issue
I would like to use CMake's FetchContent with JSBSim.
Unfortunately, since libJSBSim does not specify its include directories, this is not possible.

What is the current behavior?
libJSBSim headers could not be found.

What is the expected behavior?
CMake can correctly find the headers using the supplied include directories.

What is the motivation / use case for changing the behavior?
libJSBSim does not specify the include directories (target_include_directories).

Please tell us about your environment:

• OS: Windows 10
• JSBSim: v1.2.0
• Language: C++

Other information
The bug is simple to resolve.
I'll create a PR ASAP