package/mosh: fix dependencies

mosh selects protobuf, so it needs to propagate *all* its dependencies.

Fixes:
    http://autobuild.buildroot.org/results/1e7/1e7d28018252fa48ee1ccd68d0a21afb3e0f072f/
    http://autobuild.buildroot.org/results/a91/a91ea29454a6f8eefc0bf4f0eb137dd8e061f028/
    http://autobuild.buildroot.org/results/ecb/ecb67c1d1b04a79ad9fbe4a09121ec492fe1501c/


    [...]

[Thomas: add missing C++ comment.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>