src/common/compat_libevent.h: include testsupport.h

When tor is configured with --enable-bufferevents, the build fails
because compat_libevent.h makes use of the macro MOCK_DECL() which
is defined in testsupport.h, but not included.  We add the include.

src/common/compat_libevent.h

@@ -5,6 +5,7 @@
 #define TOR_COMPAT_LIBEVENT_H
 
 #include "orconfig.h"
+#include "testsupport.h"
 
 struct event;
 struct event_base;