Fix compilation on NetBSD/x64.

Thanks to Justin Cormack.

src/lj_alloc.c

@@ -188,7 +188,7 @@ static LJ_AINLINE void *CALL_MMAP(size_t size)
   return ptr;
 }
 
-#elif LJ_TARGET_OSX || defined(__FreeBSD__) || defined(__FreeBSD_kernel__) || defined(__OpenBSD__) || defined(__sun__)
+#elif LJ_TARGET_OSX || defined(__FreeBSD__) || defined(__FreeBSD_kernel__) || defined(__NetBSD__) || defined(__OpenBSD__) || defined(__sun__)
 
 /* OSX and FreeBSD mmap() use a naive first-fit linear search.
 ** That's perfect for us. Except that -pagezero_size must be set for OSX,