chrony plugin: constify 2 function arguments

chrony plugin: constify 2 function arguments

Fixes a couple of dozen of these warnings:
chrony.c:889:20: warning: passing 'const char [14]' to parameter of type
'char *' discards qualifiers
[-Wincompatible-pointer-types-discards-qualifiers]
  chrony_push_data("clock_stratum", "chrony", (__extension__ ({ unsigned
short int __v, __x = (unsigned short int)
(chrony_resp.body.tracking.f_stratum); if (__builtin_constant_p (__x))
__v = ((unsigned short int) ((((__x) >> 8) & 0xff) | (((__x) & 0xff) <<
8))); else __asm__ ("rorw $8, %w0" : "=r" (__v) : "0" (__x) : "cc");
__v; })));
                   ^~~~~~~~~~~~~~~