build: C++20 compatibility

A small fix to remove "volatile ++" build warning.
Source: crankyoldgit#2040

src/IRrecv.cpp

@@ -219,7 +219,7 @@ static void USE_IRAM_ATTR gpio_intr() {
     else
       params.rawbuf[rawlen] = (now - start) / kRawTick;
   }
-  params.rawlen++;
+  params.rawlen = params.rawlen + 1;  // C++20 fix
 
   start = now;
 
@@ -540,8 +540,8 @@ void IRrecv::crudeNoiseFilter(decode_results *results, const uint16_t floor) {
       for (uint16_t i = offset + 2; i <= results->rawlen && i < kBufSize; i++)
         results->rawbuf[i - 2] = results->rawbuf[i];
       if (offset > 1) {  // There is a previous pair we can add to.
-        // Merge this pair into into the previous space.
-        results->rawbuf[offset - 1] += addition;
+        // Merge this pair into into the previous space. // C++20 fix applied
+        results->rawbuf[offset - 1] = results->rawbuf[offset - 1] + addition;
       }
       results->rawlen -= 2;  // Adjust the length.
     } else {