[core] Avoid re-defining C++ round() method

[szupi-ipuzs]

I've recently been hit by the same compilation error as described here.
The easiest way to solve this is to only define this macro if the header is included from within pure C code.

Works for me :)

[szupi-ipuzs]

Sorry the linked issue is not the correct one. Now I can't find it again...

[szupi-ipuzs]

Still can't find the issue, but here's the compilation log I get without this change:

In file included from /config/.esphome/platformio/packages/toolchain-gccarmnoneeabi/arm-none-eabi/include/c++/10.3.1/mutex:39,
                 from .piolibdeps/3ch-usb-relay-1/ESPAsyncWebServer/src/AsyncEventSource.h:11,
                 from .piolibdeps/3ch-usb-relay-1/ESPAsyncWebServer/src/ESPAsyncWebServer.h:1209,
                 from src/esphome/components/web_server_base/web_server_base.h:11,
                 from src/esphome/components/captive_portal/captive_portal.h:11,
                 from src/esphome/components/captive_portal/captive_portal.cpp:1:
/config/.esphome/platformio/packages/toolchain-gccarmnoneeabi/arm-none-eabi/include/c++/10.3.1/chrono:360:47: error: macro "round" passed 2 arguments, but takes just 1
  360 |       round(const duration<_Rep, _Period>& __d)
      |                                               ^
In file included from /libretiny/cores/common/arduino/src/Arduino.h:35,
                 from <command-line>:
/libretiny/cores/common/arduino/src/wiring/wiring_compat.h:25: note: macro "round" defined here
   25 | #define round(x)       ((x) >= 0 ? (long)((x) + 0.5) : (long)((x) - 0.5))
      |