net: websocket: Fix truncated string warning on copying

WS_MAGIC is a constant string and when calculating lengths for copying
we always exclude the NULL terminator. In result, using strncpy() for
copying can generate a warning about truncated string, as WS_MAGIC will
always be truncated from the NULL terminator. Therefore replace
strncpy() with memcpy() as it seems more appropriate for this case.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>