tests/Makefile.am: clean png files

tests/Makefile.am

@@ -60,7 +60,7 @@ EXTRA_DIST =					\
 	microbenchs/microbench.sh
 
 CLEANFILES = 					\
-	*.gcno *.gcda *.linkinfo core starpu_idle_microsec.log *.mod
+	*.gcno *.gcda *.linkinfo core starpu_idle_microsec.log *.mod *.png
 
 BUILT_SOURCES =
 SUBDIRS =