[DCCPv6]: Resolve conditional build problem

Resolves the problem that if IPv6 was configured `y' and DCCP `m' then
dccp_ipv6 was not built as a module. With this change, dccp_ipv6 is built
as a module whenever DCCP *OR* IPv6 are configured as modules; it will be
built-in only if both DCCP = `y' and IPV6 = `y'.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

net/dccp/Makefile

-obj-$(CONFIG_IPV6) += dccp_ipv6.o
-
-dccp_ipv6-y := ipv6.o
-
 obj-$(CONFIG_IP_DCCP) += dccp.o dccp_ipv4.o
 
 dccp-y := ccid.o feat.o input.o minisocks.o options.o output.o proto.o timer.o
 
 dccp_ipv4-y := ipv4.o
 
+# build dccp_ipv6 as module whenever either IPv6 or DCCP is a module
+obj-$(subst y,$(CONFIG_IP_DCCP),$(CONFIG_IPV6)) += dccp_ipv6.o
+dccp_ipv6-y := ipv6.o
+
 dccp-$(CONFIG_IP_DCCP_ACKVEC) += ackvec.o
 
 obj-$(CONFIG_INET_DCCP_DIAG) += dccp_diag.o