Use target_include_directories to specify public header path

[ChrisThrasher]

discord-rpc/src/CMakeLists.txt

Line 1 in 963aa9f

include_directories(${PROJECT_SOURCE_DIR}/include)

This use of include_directories means that those who link to the discord-rpc target will not inherit the include paths required to use this target. An easy fix is to change this to a target_include_directories call with PUBLIC visibility so those linking to discord-rpc don't need to manually specify the include paths.

[p0358]

This was PR'd into our fork of this library: harmonytf#8