Externals: Add cpp-ipc subdirectory with EXCLUDE_FROM_ALL.

The cpp-ipc dependency was included in #13870; it was overlooked that
`install()` commands in the library would lead to ancillary files being
installed along side Dolphin on Linux.

`EXCLUDE_FROM_ALL` is now set in the `add_subdirectory()` call to
prevent cpp-ipc from being part of the installation.

In particular, the following files should no longer be seen in the build
logs or in the final builds:

```
-- Installing: /app/include/libipc
-- Installing: /app/include/libipc/condition.h
-- Installing: /app/include/libipc/buffer.h
-- Installing: /app/include/libipc/export.h
-- Installing: /app/include/libipc/def.h
-- Installing: /app/include/libipc/rw_lock.h
-- Installing: /app/include/libipc/shm.h
-- Installing: /app/include/libipc/mutex.h
-- Installing: /app/include/libipc/pool_alloc.h
-- Installing: /app/include/libipc/ipc.h
-- Installing: /app/include/libipc/semaphore.h
-- Installing: /app/lib/libipc.a
-- Installing: /app/share/cpp-ipc/cpp-ipc-targets.cmake
-- Installing: /app/share/cpp-ipc/cpp-ipc-targets-release.cmake
-- Installing: /app/share/cpp-ipc/cpp-ipc-config.cmake
-- Installing: /app/share/cpp-ipc/cppIpcConfigVersion.cmake
```

Externals/cpp-ipc/CMakeLists.txt

@@ -1,4 +1,4 @@
-add_subdirectory(cpp-ipc)
+add_subdirectory(cpp-ipc EXCLUDE_FROM_ALL)
 
 dolphin_disable_warnings(ipc)