samba: add option to install libsmbclient

Closes #301

The libsmbclient library is part of the samba package. It can be used
to build applications that access Samba shares without the need to
mount them. Currently the library is explicitly not installed by
buildroot. Here's a patch that adds a config option to install it.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>