Mac build break caused by include/posix/io_posix.h not declearing errno,

Summary: Mac build breaks as include/posix/io_posix.h doesn't include errno. Move the exact function declaration to io_posix.cc

Test Plan: Run all test. Will run on Mac

Reviewers: rven, anthony, yhchiang, IslamAbdelRahman, igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D49551

include/posix/io_posix.h

@@ -164,14 +164,8 @@ class PosixMmapFile : public WritableFile {
 class PosixDirectory : public Directory {
  public:
   explicit PosixDirectory(int fd) : fd_(fd) {}
-  ~PosixDirectory() { close(fd_); }
-
-  virtual Status Fsync() override {
-    if (fsync(fd_) == -1) {
-      return IOError("directory", errno);
-    }
-    return Status::OK();
-  }
+  ~PosixDirectory();
+  virtual Status Fsync() override;
 
  private:
   int fd_;

util/io_posix.cc

@@ -633,5 +633,14 @@ size_t PosixWritableFile::GetUniqueId(char* id, size_t max_size) const {
   return GetUniqueIdFromFile(fd_, id, max_size);
 }
 #endif
+
+PosixDirectory::~PosixDirectory() { close(fd_); }
+
+Status PosixDirectory::Fsync() {
+  if (fsync(fd_) == -1) {
+    return IOError("directory", errno);
+  }
+  return Status::OK();
+}
 }  // namespace rocksdb
 #endif