Add missing include for flock()

Summary: flock() expects sys/file.h which on many platforms
is getting included as a side-effect, but on at least one
this isn't the case.

Add the explicit include to fix builds on those platforms.

Closes #232

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Unrelated runtime failures.

Reviewed By: @JoelMarcey

Differential Revision: D2196306

folly/File.cpp

@@ -18,6 +18,7 @@
 
 #include <fcntl.h>
 #include <unistd.h>
+#include <sys/file.h>
 
 #include <folly/Exception.h>
 #include <folly/FileUtil.h>