net/libutp: unbreak build with Clang 6 (C++14 by default)

c++ -O2 -pipe -fstack-protector -fno-strict-aliasing -fno-exceptions -fno-rtti -I/wrkdirs/usr/ports/net/libutp/work/libutp-7c4f19a -I/wrkdirs/usr/ports/net/libutp/work/libutp-7c4f19a/utp_config_lib -DPOSIX -Wall -Wno-c++11-extensions -c utp.cpp -o utp.o
utp.cpp:1708:91: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]

"scaled_gain:%f rtt:%u rate:%u quota:%d wnduser:%u rto:%u timeout:%d get_microseconds:"I64u" "
                                                                                       ^

utp.cpp:1793:79: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]

LOG_UTPV("0x%08x: Got %s. seq_nr:%u ack_nr:%u state:%s version:%u timestamp:"I64u" reply_micro:%u",
                                                                             ^

Reported by: antoine (via bug 224669)