net-im/mtxclient: Bump PORTREVISION to fix net-im/nheko build

After the devel/nlohmann-json update in 551e034565, net-im/nheko fails
to build due to linking errors unless net-im/mtxclient is rebuilt.

ld: error: undefined symbol:
mtx::events::state::to_json(nlohmann::json_abi_v3_11_3::basic_json<std::1::map,
std::1::vector, std::1::basic_string<char,
std::1::char_traits<char>, std::1::allocator<char>>, bool, long,
unsigned long, double, std::1::allocator,
nlohmann::json_abi_v3_11_3::adl_serializer, std::1::vector<unsigned
char, std::1::allocator<unsigned char>>, void>&,
mtx::events::state::Aliases const&) >>> referenced by EventStore.cpp

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42886