package/espeak: add comment to ignore 'unmet dependencies'

On master 86b4eecc, espeak can generate spurious 'unmet dependencies'
messages:

    $ make KCONFIG_SEED=0x7A85DEE0 randconfig
    warning: (BR2_PACKAGE_ESPEAK_AUDIO_BACKEND_ALSA) selects
    BR2_PACKAGE_PORTAUDIO_CXX which has unmet directdependencies
    (BR2_PACKAGE_PORTAUDIO && BR2_INSTALL_LIBSTDCPP)

However, the dpendency chain *is* correct. There is something that
causes the kconfig parser to get really confused...

Add a comment statng the issue is spurious, so noone tries to fix it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnaud Aujon <arnaud@intelibre.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>