net/libtrace: Fix build with Clang 6

tracetop.cc:380:16: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]

printw("%7"PRIu64"\t%7"PRIu64"\n",
           ^

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/libtrace-3.0.19_2.log