hv_netvsc: select needed ucs2_string routine

The conversion of rndis friendly name to utf8 uses a standard
kernel routine which is optional in config. Therefore build
would fail for some configurations. Resolve by selecting needed
library.

Fixes: 0fe554a4 ("hv_netvsc: propogate Hyper-V friendly name into interface alias")
Signed-off-by: NStephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/hyperv/Kconfig

 config HYPERV_NET
 	tristate "Microsoft Hyper-V virtual network driver"
 	depends on HYPERV
+	select UCS2_STRING
 	help
 	  Select this option to enable the Hyper-V virtual network driver.