mptcp: MPTCP_IPV6 should depend on IPV6 instead of selecting it

MPTCP_IPV6 selects IPV6, thus enabling an optional feature the user may
not want to enable.  Fix this by making MPTCP_IPV6 depend on IPV6, like
is done for all other IPv6 features.

Fixes: f870fa0b ("mptcp: Add MPTCP socket stubs")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20201020073839.29226-1-geert@linux-m68k.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/Kconfig

@@ -19,7 +19,7 @@ config INET_MPTCP_DIAG
 
 config MPTCP_IPV6
 	bool "MPTCP: IPv6 support for Multipath TCP"
-	select IPV6
+	depends on IPV6
 	default y
 
 config MPTCP_KUNIT_TESTS