Fix OSX build break after the fallocate change

Summary:
The recent update about fallocate failed OSX build. Fix it.
Closes https://github.com/facebook/rocksdb/pull/1830

Differential Revision: D4500235

Pulled By: siying

fbshipit-source-id: a5f2b40

util/io_posix.cc

@@ -757,6 +757,7 @@ Status PosixWritableFile::Allocate(uint64_t offset, uint64_t len) {
 }
 #endif
 
+#ifdef OS_LINUX
 Status PosixWritableFile::RangeSync(uint64_t offset, uint64_t nbytes) {
   assert(offset <= std::numeric_limits<off_t>::max());
   assert(nbytes <= std::numeric_limits<off_t>::max());
@@ -771,6 +772,7 @@ Status PosixWritableFile::RangeSync(uint64_t offset, uint64_t nbytes) {
 size_t PosixWritableFile::GetUniqueId(char* id, size_t max_size) const {
   return PosixHelper::GetUniqueIdFromFile(fd_, id, max_size);
 }
+#endif
 
 /*
  * PosixRandomRWFile

util/io_posix.h

@@ -114,8 +114,10 @@ class PosixWritableFile : public WritableFile {
 #ifdef ROCKSDB_FALLOCATE_PRESENT
   virtual Status Allocate(uint64_t offset, uint64_t len) override;
 #endif
+#ifdef OS_LINUX
   virtual Status RangeSync(uint64_t offset, uint64_t nbytes) override;
   virtual size_t GetUniqueId(char* id, size_t max_size) const override;
+#endif
 };
 
 // mmap() based random-access