MAP_POPULATE is Linux-specific and not declared on NetBSD

https://www.man7.org/linux/man-pages/man2/mmap.2.html
https://man.netbsd.org/mmap.2

Signed-off-by: leleliu008 <leleliu008@gmail.com>

src/file_reader.cpp

@@ -143,12 +143,12 @@ class MMapException : std::exception {};
 char*
 mmapReadOnly(int fd, offset_type offset, size_type size)
 {
-#if defined(__APPLE__) || defined(__OpenBSD__) || defined(__HAIKU__)
- const auto MAP_FLAGS = MAP_PRIVATE;
+#if defined(__linux__)
+ const auto MAP_FLAGS = MAP_PRIVATE|MAP_POPULATE;
 #elif defined(__FreeBSD__)
  const auto MAP_FLAGS = MAP_PRIVATE|MAP_PREFAULT_READ;
 #else
- const auto MAP_FLAGS = MAP_PRIVATE|MAP_POPULATE;
+ const auto MAP_FLAGS = MAP_PRIVATE;
 #endif
 
  const auto p = (char*)mmap(NULL, size, PROT_READ, MAP_FLAGS, fd, offset);