[RocksDB] posix_logger does not compile on non-linux platform

Summary: As title. Found out this when testing stack_trace.cc portability.

Test Plan: make check; manual test 'non-linux' build by forcing OS_LINUX2

Reviewers: dhruba, heyongqiang

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10263

util/posix_logger.h

@@ -94,12 +94,12 @@ class PosixLogger : public Logger {
       }
 
       assert(p <= limit);
+      const size_t write_size = p - base;
 
 #ifdef OS_LINUX
       // If this write would cross a boundary of kDebugLogChunkSize
       // space, pre-allocate more space to avoid overly large
       // allocations from filesystem allocsize options.
-      const size_t write_size = p - base;
       const int last_allocation_chunk =
         ((kDebugLogChunkSize - 1 + log_size_) / kDebugLogChunkSize);
       const int desired_allocation_chunk =