The following code is constant-time.

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

typedef uint8_t(* issue_t)(const uint8_t*, const uint8_t*);

uint8_t issue(const uint8_t *a, const uint8_t *b) {
  return !b[0]*a[0] + b[0]*a[1];
}

int main(int argc, char *argv[]) {
  uint8_t a[2] ={2, 5}, b[2] = {0};
  // b is our secret value containing array, we read its content from the 
  // environment so the compiler cannot play tricks on us by inlining
  if (argc >= 2) {
    b[0] = (uint8_t) strtol(argv[1], NULL, 16);
  }
  else {
    fprintf(stderr, "USAGE: %s hex_value\n", argv[0]);
    return 1;
  }
  // make sure b[0] is either 0 or 1
  b[0] = !!b[0];
  // declarations and markup here
  
  volatile issue_t func = issue;
  printf("%d\n", func(a, b));

  return 0;
}

See the compilation results with different optimization levels on GodBolt.