RemoteVstPlugin: add include directory

If libraries are being used from somewhere other than
/usr/include or /usr/local/include, building vst_remote would fail.
Now it checks CMAKE_INSTALL_PREFIX/include.
(cherry picked from commit 7d0c1ef6c2)
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f