samba: remove swat documentation when not needed

When SWAT (the Web administration tool of Samba) is enabled, which is
the default when one enables samba in Buildroot, a lot of
documentation gets installed in /usr/swat (~15 MB). This patch fixes
this by removing the documentation when BR2_HAVE_DOCUMENTATION is not
set.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>