clients: Include and link against math lib for new multi-resource

Linking failed with missing ceil() here. Making sure that we include
the header and add the missing -lm.

Signed-off-by: Stefan Schmidt <s.schmidt@samsung.com>
diff --git a/clients/multi-resource.c b/clients/multi-resource.c
index 1c2e5c9..6f93618 100644
--- a/clients/multi-resource.c
+++ b/clients/multi-resource.c
@@ -35,6 +35,7 @@
 #include <time.h>
 #include <sys/poll.h>
 #include <float.h>
+#include <math.h>
 
 #include <wayland-client.h>
 #include "../shared/os-compatibility.h"