net: lib: ptp: Fix `PTP_UDP_IPv6_PROTOCOL` compilation error

When the `PTP_UDP_IPv6_PROTOCOL` configuration is enabled
the `mcast_addr` variable is defined as an IPv6 structure, which
doesn't have `s_addr` member.

To not introduce preprocessor guarded code blocks in functions
added union to represent both configurations: IPv4 and IPv6.

Fixes: #92975

Signed-off-by: Adam Wojasinski <awojasinski@baylibre.com>