Shrink Enum Sizes

Passed STC:
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 110848 W: 28974 L: 28564 D: 53310
Ptnml(0-2): 302, 12118, 30132, 12612, 260
https://tests.stockfishchess.org/tests/view/68242770a527315e07ccca38

closes https://github.com/official-stockfish/Stockfish/pull/6063

no functional change

src/timeman.cpp

@@ -28,6 +28,8 @@
 
 namespace Stockfish {
 
+enum Color : int8_t;
+
 TimePoint TimeManagement::optimum() const { return optimumTime; }
 TimePoint TimeManagement::maximum() const { return maximumTime; }