Don't use lstrlenW() on Windows

The lstrlenW() function isn't available to UWP apps, and isn't necessary, since
when given -1, WideCharToMultiByte() will process the terminating null character
itself (and the length returned by the function includes this character).

Resolves #2994

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>

library/x509_crt.c

@@ -1449,8 +1449,8 @@ int mbedtls_x509_crt_parse_path(mbedtls_x509_crt *chain, const char *path)
         }
 
         w_ret = WideCharToMultiByte(CP_ACP, 0, file_data.cFileName,
-                                    lstrlenW(file_data.cFileName),
-                                    p, (int) len - 1,
+                                    -1,
+                                    p, (int) len,
                                     NULL, NULL);
         if (w_ret == 0) {
             ret = MBEDTLS_ERR_X509_FILE_IO_ERROR;