core: add nm_utils_get_monotonic_timestamp_us() function

Signed-off-by: Thomas Haller <thaller@redhat.com>

src/NetworkManagerUtils.h

@@ -97,6 +97,7 @@ NMConnection *nm_utils_match_connection (GSList *connections,
 gint64 nm_utils_ascii_str_to_int64 (const char *str, guint base, gint64 min, gint64 max, gint64 fallback);
 
 #define NM_UTILS_NS_PER_SECOND  ((gint64) 1000000000)
+gint64 nm_utils_get_monotonic_timestamp_us (void);
 gint64 nm_utils_get_monotonic_timestamp_ms (void);
 gint32 nm_utils_get_monotonic_timestamp_s (void);