treewide: use boost::lround when std::round is unavailable

This is the case with uClibc-ng currently.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
(cherry picked from commit 769cd0ee9f0cf8ceb026aa751b5d4a390bb5dbdc)
(changed define to match master)

src/mixer/plugins/WinmmMixerPlugin.cxx

@@ -20,13 +20,13 @@
 #include "mixer/MixerInternal.hxx"
 #include "output/OutputAPI.hxx"
 #include "output/plugins/WinmmOutputPlugin.hxx"
+#include "util/Math.hxx"
 
 #include <mmsystem.h>
 
 #include <stdexcept>
 
 #include <assert.h>
-#include <math.h>
 #include <windows.h>
 
 class WinmmMixer final : public Mixer {