replay_gain_info, ...: use cmath instead of math.h in C++ mode

Fixes build problems with mingw32.

src/PlayerThread.cxx

@@ -37,6 +37,8 @@ extern "C" {
 #include "idle.h"
 }
 
+#include <cmath>
+
 #include <glib.h>
 
 #undef G_LOG_DOMAIN
@@ -759,7 +761,7 @@ play_next_chunk(struct player *player)
 						  other_chunk->tag);
 			other_chunk->tag = NULL;
 
-			if (isnan(pc->mixramp_delay_seconds)) {
+			if (std::isnan(pc->mixramp_delay_seconds)) {
 				chunk->mix_ratio = ((float)cross_fade_position)
 					     / player->cross_fade_chunks;
 			} else {