devel/vxlog: Fix build with Clang 6

src/vxlog/type-to-string.h:65:37: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]

int res = snprintf(str, len, "%"PRId64, static_cast<int64_t>(value));
                                ^

http://beefy11.nyi.freebsd.org/data/head-i386-default/p473363_s335650/logs/vxlog-1.0.3_1.log