Fix 'make strip' for mingw.

Currently the make strip target is broken on mingw as the exe name is wrong (stockfish instead of stockfish.exe).

Needs some testing by mingw users (both profile-build and strip, native and cross).

No functional change.

src/Makefile

@@ -22,7 +22,11 @@
 ### ==========================================================================
 
 ### Executable name
+ifeq ($(COMP),mingw)
+EXE = stockfish.exe
+else
 EXE = stockfish
+endif
 
 ### Installation dir definitions
 PREFIX = /usr/local
@@ -437,7 +441,7 @@ clean: objclean profileclean
 
 # clean binaries and objects
 objclean:
-	@rm -f $(EXE) $(EXE).exe *.o ./syzygy/*.o
+	@rm -f $(EXE) *.o ./syzygy/*.o
 
 # clean auxiliary profiling files
 profileclean: