Documentation: Make doxygen-internal depend on public inputs

The internal documentation is a superset of the public one, so make the
`doxygen-internal` target depend on the public input files. Otherwise
the internal documentation is not rebuilt if a "public" file changes.

Before:

 $ touch src/libcamera/camera.cpp
 $ ninja -C build -v
 # ...
 [6/116] /usr/bin/doxygen Documentation/Doxyfile-public
 # ...

After:

 $ touch src/libcamera/camera.cpp
 $ ninja -C build -v
 # ...
 [6/117] /usr/bin/doxygen Documentation/Doxyfile-public
 # ...
 [9/117] /usr/bin/doxygen Documentation/Doxyfile-internal
 # ...

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Documentation/meson.build

@@ -103,6 +103,7 @@ if doxygen.found() and dot.found()
                   input : [
                       doxyfile,
                       doxyfile_common,
+                      doxygen_public_input,
                       doxygen_internal_input,
                   ],
                   output : 'internal-api-html',