IAR declares strncasecmp in string.h not strings.h

strings.h is not an ISO header file, so IAR generates fatal error Pe1696
'cannot open source file "strings.h"'. Even though strncasecmp isn't an
ISO C library function, IAR's runtime library defines it, though it
declares it in string.h instead.

examples/device/usbtmc/src/usbtmc_app.c

@@ -23,7 +23,11 @@
  *
  */
 
+#ifdef __ICCARM__
+#include <string.h>
+#else
 #include <strings.h>
+#endif
 #include <stdlib.h>     /* atoi */
 #include "tusb.h"
 #include "bsp/board.h"