Windows: MSVC2017: adapt testsource plugin module

plugins/samplesource/testsource/testsourcethread.cpp

@@ -14,6 +14,8 @@
 // along with this program. If not, see <http://www.gnu.org/licenses/>.          //
 ///////////////////////////////////////////////////////////////////////////////////
 
+#define _USE_MATH_DEFINES
+#include <math.h>
 #include <stdio.h>
 #include <errno.h>
 #include "testsourcethread.h"