[build] define rsize_t on FreeBSD

(unsure why lighttpd CI CMake build has rsize_t defined,
 but not autotools or SCons lighttpd CI builds)

src/ck.h

@@ -14,6 +14,10 @@
 #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