use pio native to build standalone

lib_standalone/emsuart_standalone.cpp

@@ -16,6 +16,8 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#ifdef EMSESP_STANDALONE
+
 #include "emsuart_standalone.h"
 
 #pragma GCC diagnostic push
@@ -86,3 +88,5 @@ char * EMSuart::hextoa(char * result, const uint8_t value) {
 } // namespace emsesp
 
 #pragma GCC diagnostic pop
+
+#endif