[build] define rsize_t on FreeBSD
(unsure why lighttpd CI CMake build has rsize_t defined,
but not autotools or SCons lighttpd CI builds)
#endif
#include "first.h"
#ifdef __FreeBSD__
+#ifndef _RSIZE_T_DEFINED /* expecting __EXT1_VISIBLE 1 and _RSIZE_T_DEFINED */
+#define _RSIZE_T_DEFINED
+typedef size_t rsize_t;
+#endif
#include <errno.h>
#endif
