CLEANUP: freq_ctr: make arguments of freq_ctr_total() const

freq_ctr_total() doesn't modify the freq counters, it should take a
const argument.

include/haproxy/freq_ctr.h

@@ -29,7 +29,7 @@
 #include <haproxy/time.h>
 
 /* exported functions from freq_ctr.c */
-ullong freq_ctr_total(struct freq_ctr *ctr, uint period, int pend);
+ullong freq_ctr_total(const struct freq_ctr *ctr, uint period, int pend);
 
 /* Update a frequency counter by <inc> incremental units. It is automatically
  * rotated if the period is over. It is important that it correctly initializes

src/freq_ctr.c

@@ -25,7 +25,7 @@
  * read_freq_ctr_period() to avoid reporting ups and downs on low-frequency
  * events when the past value is <= 1.
  */
-ullong freq_ctr_total(struct freq_ctr *ctr, uint period, int pend)
+ullong freq_ctr_total(const struct freq_ctr *ctr, uint period, int pend)
 {
 	ullong curr, past;
 	uint curr_tick;