calibrator.cpp: include <climits> for PATH_MAX if needed
Needed to get the definition of PATH_MAX
included on Solaris 11.4.
Without this change, building on Solaris failed with:
calibrator.cpp: In member function ‘bool Calibrator::is_sysfs_name(const char*)’:
calibrator.cpp:215:27: error: ‘PATH_MAX’ was not declared in this scope
215 | char filename[PATH_MAX]; // actually 35, but hey...
| ^~~~~~~~