build: Silence clang warning on older glib autoptr usage

build: Silence clang warning on older glib autoptr usage

glib's G_DEFINE_AUTOPTR_CLEANUP_FUNC() macro defines several static
inline functions, often with some of them unused, but prior to 2.57.2
did not mark the functions as such.  As a result, clang (but not gcc)
fails to build with older glib unless -Wno-unused-function is enabled.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20200317175534.196295-1-eblake@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>