Fix sim-eng build.

strerror() must be declared (include <string.h>).

Change-Id: I0e93874d1bb7dad113f7c82d0ff1b5e6733b36c1
diff --git a/tests/nusensors/nusensors.cpp b/tests/nusensors/nusensors.cpp
index 731fa09..29e2069 100644
--- a/tests/nusensors/nusensors.cpp
+++ b/tests/nusensors/nusensors.cpp
@@ -15,6 +15,7 @@
  */
 
 #include <stdint.h>
+#include <string.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>