chelper: Use CLOCK_MONOTONIC_RAW

Switch the host code from the CLOCK_MONOTONIC clock to the Linux
specific CLOCK_MONOTONIC_RAW clock.  It's common for ntp to slew the
CLOCK_MONOTONIC clock to account for drift, and that can break the
host's ability to make accurate predictions of the micro-controller
clock.  This could lead to "move queue empty" errors.  The
CLOCK_MONOTONIC_RAW clock is not slewed and thus should not have this
problem.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/chelper/pyhelper.c

@@ -18,7 +18,7 @@ double __visible
 get_monotonic(void)
 {
     struct timespec ts;
-    int ret = clock_gettime(CLOCK_MONOTONIC, &ts);
+    int ret = clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
     if (ret) {
         report_errno("clock_gettime", ret);
         return 0.;