symbolizer: Fix missing unistd.h include.

Summary:
Fixes `close`/`lseek`/`read`/`pread` not being in scope.

It only worked so far because `#include <glog/logging.h>` apparently brought them into scope; with glog >= 0.7.0 that does not work anymore.

See also facebook/folly#2171 (comment)

X-link: facebook/folly#2245

Reviewed By: Gownta

Differential Revision: D58968225

Pulled By: Orvid

fbshipit-source-id: fe9b4ba3d08e910b60225d3ed6b17a314655dfe4

third-party/folly/src/folly/debugging/symbolizer/Elf.cpp

@@ -30,6 +30,7 @@
 #include <folly/lang/CString.h>
 #include <folly/portability/Config.h>
 #include <folly/portability/SysMman.h>
+#include <folly/portability/Unistd.h>
 
 #if FOLLY_HAVE_ELF