devel/gearmand-devel: fix build with GCC-based architectures

libtest/timer.hpp needs to include stdint.h for uint64.

./libtest/timer.hpp:55:36: error: 'int64_t' has not been declared

void offset(int64_t minutes_arg, int64_t seconds_arg, int64_t nanoseconds);
                                 ^~~~~~~

PR: 237153
Approved by: bdrewery (maintainer timeout), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20114