host_memory: allow missing MAP_NORESERVE on FreeBSD after de66cf5637

src/common/host_memory.cpp:408:47: error: use of undeclared identifier 'MAP_NORESERVE'
                MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
                                              ^

src/common/host_memory.cpp

@@ -25,6 +25,10 @@
 #include <unistd.h>
 #include "common/scope_exit.h"
 
+#ifndef MAP_NORESERVE
+#define MAP_NORESERVE 0
+#endif
+
 #endif // ^^^ Linux ^^^
 
 #include <mutex>