selftests: net: ip_local_port_range: define IPPROTO_MPTCP

Older glibc's netinet/in.h may leave IPPROTO_MPTCP undefined when
building ip_local_port_range.c, that leads to "error: use of undeclared
identifier 'IPPROTO_MPTCP'".

Define IPPROTO_MPTCP in such cases, just like in other MPTCP selftests.

Fixes: 122db5e3

 ("selftests/net: add MPTCP coverage for IP_LOCAL_PORT_RANGE")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Closes: https://lore.kernel.org/netdev/CA+G9fYvGO5q4o_Td_kyQgYieXWKw6ktMa-Q0sBu6S-0y3w2aEQ@mail.gmail.com/
Signed-off-by: Maxim Galaganov <max@internet.ru>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Link: https://lore.kernel.org/r/20240209132512.254520-1-max@internet.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>