integration fix: have a central place of definition for clock_difftime_ns

Prior to this commit, the following command failed to build dlsim:
    ./build_oai --phy_simulators -c

Choice has been made to define it in a .h file as a static inline
function.

common/utils/time_utils.h

@@ -0,0 +1,23 @@
+#ifndef _TIME_UTILS_H_
+#define _TIME_UTILS_H_
+
+#include <time.h>
+#include <stdint.h>
+
+static inline int64_t clock_difftime_ns(struct timespec start, struct timespec end)
+{
+  struct timespec temp;
+  int64_t temp_ns;
+
+  if ((end.tv_nsec-start.tv_nsec)<0) {
+    temp.tv_sec = end.tv_sec-start.tv_sec-1;
+    temp.tv_nsec = 1000000000+end.tv_nsec-start.tv_nsec;
+  } else {
+    temp.tv_sec = end.tv_sec-start.tv_sec;
+    temp.tv_nsec = end.tv_nsec-start.tv_nsec;
+  }
+  temp_ns = (int64_t)(temp.tv_sec) * (int64_t)1000000000 + (temp.tv_nsec);
+  return temp_ns;
+}
+
+#endif /* _TIME_UTILS_H_ */