Externals: Update mGBA to latest master.

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
```

Co-authored-by: Joshua Vandaële <joshua@vandaele.software>

The bug in CVE-2025-0665 causes crashes on Android 11 and above, as the default fdsan behavior was changed to abort when a violation is found.

While curl 8.17.0 is the latest release, we can't upgrade to it right now due to mbedtls 2.28.0 (which we are currently using) being incompatible with it.

Co-Authored-By: OatmealDome <OatmealDome@users.noreply.github.com>

Externals: Make imgui a submodule

The last official release is, at the time of writing, almost two years
old. The latest development snapshot (updated ~2 months ago) will be
used.