[spinel] fix use of OT header file in spinel.c (#5141)

`spinel.h` and `spinel.c` are soruce files that may be used by host
side drivers and therefore the souurce code itself should not use any
of OT header files. Spienl file provide `SPINEL_PLATFORM_HEADER` to
allow platform to add their own includes if they want to.

src/lib/spinel/spinel.c

@@ -54,7 +54,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
-#include "utils/wrap_string.h"
+#include <string.h>
 #endif // #ifndef SPINEL_PLATFORM_HEADER
 
 // ----------------------------------------------------------------------------