Track seldepth in qsearch too

Sometimes if we count the reported PV length, it turns out to be longer than the
selective depth reported. This fixes this behavior by applying the selective
depth to qsearch since we do report PVs from it as well.

Passed non-regression STC:
https://tests.stockfishchess.org/tests/view/656cf5b66980e15f69c7499d
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 223648 W: 56372 L: 56356 D: 110920
Ptnml(0-2): 710, 25580, 59231, 25590, 713

closes official-stockfish#4903

No functional change

src/search.cpp

@@ -1421,6 +1421,10 @@ Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
     ss->inCheck        = pos.checkers();
     moveCount          = 0;
 
+    // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
+    if (PvNode && thisThread->selDepth < ss->ply + 1)
+        thisThread->selDepth = ss->ply + 1;
+
     // Step 2. Check for an immediate draw or maximum ply reached
     if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
         return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;