Avoid unused-variable warnings for str as well

The exact guard is FS_IO && PK_PARSE_C. Just keep it simple.

tests/suites/test_suite_pk.function

@@ -200,15 +200,14 @@ void invalid_parameters( )
     unsigned char *null_buf = NULL;
     unsigned char buf[1];
     unsigned char *p = buf;
-#if defined(MBEDTLS_FS_IO)
     char str[1] = {0};
-#endif
     mbedtls_pk_context pk;
     mbedtls_md_type_t valid_md = MBEDTLS_MD_SHA256;
     void *options = buf;
 
     (void) null_buf;
     (void) p;
+    (void) str;
 
     mbedtls_pk_init( &pk );