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

@@ -0,0 +1,6 @@
+CFLAGS=-Wall --pedantic-errors -O2 -static
+
+all: get_clock_boottime
+
+get_clock_boottime: get_clock_boottime.c
+	$(CC) $(CFLAGS) $^ -o $@

src/get_clock_boottime/get_clock_boottime.c

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+int main(void) {
+    int ret;
+    struct timespec tp;
+
+    ret = clock_gettime(CLOCK_BOOTTIME, &tp);
+    if (ret) {
+        perror("clock_gettime()");
+        return EXIT_FAILURE;
+    }
+
+    printf("%ld.%ld\n", tp.tv_sec, tp.tv_nsec);
+
+    return EXIT_SUCCESS;
+}