instrument/daq: Use clock boottime for the time column of the energy measurements

92e16ee873 ("instrument/daq: Add an explicit time column to the DAQ
measurements") added a time column to the DAQ measurements in order to
help correlate them with those of other collectors like
FtraceCollector.  Sadly, FTrace uses CLOCK_BOOTTIME instead of
CLOCK_MONOTONIC by default.  CLOCK_MONOTONIC is like CLOCK_BOOTTIME,
except that it stops when the device suspends, which is why I hadn't
spot the issue until now.

Switch to CLOCK_BOOTTIME to get the intended behaviour of the original
commit.

src/get_clock_boottime/Makefile

@@ -1,6 +1,6 @@
 CFLAGS=-Wall --pedantic-errors -O2 -static
 
-all: get_clock_monotonic
+all: get_clock_boottime
 
-get_clock_monotonic: get_clock_monotonic.c
+get_clock_boottime: get_clock_boottime.c
 	$(CC) $(CFLAGS) $^ -o $@

src/get_clock_boottime/get_clock_boottime.c

@@ -6,7 +6,7 @@ int main(void) {
     int ret;
     struct timespec tp;
 
-    ret = clock_gettime(CLOCK_MONOTONIC, &tp);
+    ret = clock_gettime(CLOCK_BOOTTIME, &tp);
     if (ret) {
         perror("clock_gettime()");
         return EXIT_FAILURE;