make coreclr buildable on FreeBSD again (#15480)

* small changes to get coreclr building on FreeBSD again

* include <stdlib.h> to get prototype for rand()

* remove extra line

* include copmmon.h instead of stdlib.h

Author: wfurt

src/gc/unix/events.cpp

@@ -10,6 +10,7 @@
 #include <pthread.h>
 #include <errno.h>
 #include "config.h"
+#include "common.h"
 
 #include "gcenv.structs.h"
 #include "gcenv.base.h"

src/gc/unix/gcenv.unix.cpp

@@ -8,7 +8,9 @@
 #include <memory>
 #include <pthread.h>
 #include <signal.h>
+
 #include "config.h"
+#include "common.h"
 
 #include "gcenv.structs.h"
 #include "gcenv.base.h"