samba: unconditionally enable --with-sys-quotas

Contrary to what was written in samba.mk, the sys-quotas option
apparently does not depend on RPC support in the toolchain: Samba
builds perfectly fine on an uClibc toolchain without RPC support with
sys-quotas enabled.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>