[spinel] fix when strnlen is not a macro (#4548)

If strnlen is a normal function, the current code will result in
duplicate definition error.

src/ncp/spinel.c

@@ -148,7 +148,7 @@ static int spinel_errno_workaround_;
 #endif
 
 #ifndef strnlen
-size_t strnlen(const char *s, size_t maxlen)
+static size_t spinel_strnlen(const char *s, size_t maxlen)
 {
     size_t ret;
 
@@ -159,6 +159,8 @@ size_t strnlen(const char *s, size_t maxlen)
 
     return ret;
 }
+#else
+#define spinel_strnlen strnlen
 #endif
 
 typedef struct
@@ -552,7 +554,7 @@ static spinel_ssize_t spinel_datatype_vunpack_(bool           in_place,
             // Add 1 for zero termination. If not zero terminated,
             // len will then be data_len+1, which we will detect
             // in the next check.
-            len = strnlen((const char *)data_in, data_len) + 1;
+            len = spinel_strnlen((const char *)data_in, data_len) + 1;
 
             // Verify that the string is zero terminated.
             require_action(len <= data_len, bail, (ret = -1, errno = EOVERFLOW));