Enable exporting CMake interface target

[tytan652]

Description

Required by:

• Add procedure handler based API obs-browser#431
• cmake,lib,base: Export obs-websocket-api as a target obs-websocket#1196

Motivation and Context

Needed to enable installing obs-websocket-api and obs-browser-api headers

How Has This Been Tested?

Build on Linux (still uses legacy path) with obs-browser-api PR and check if the header is installed, which is the case.

Ci for Windows and macOS, headers install needs to be checked locally

• Tested on Windows, --component Development adds the header

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.