Fix Unused Variable Error

Summary:
D68874718 causes some build issues (see https://www.internalfb.com/intern/test/562950120027900?ref_report_id=0 for an example) because the `inOutSpec` variable is not referenced in the function.

Adding the `[[maybe_unused]]` fixes this with the one test I ran, but I'll need CI to verify everything else. Let's use the macro `MAYBE_UNUSED` to be sure that this fix is supported by all compilers.

Differential Revision: D68985760

fbshipit-source-id: d3894c2e6c20055d6f512c16bd69dd47b7ac99e4

vrs/WriteFileHandler.h

@@ -16,6 +16,8 @@
 
 #pragma once
 
+#include <vrs/os/CompilerAttributes.h>
+
 #include "FileHandler.h"
 
 namespace vrs {
@@ -77,7 +79,7 @@ class WriteFileHandler : public FileHandler {
   /// @param inOutSpec: file spec used for the file creation, that will be passed to a
   /// DiskFile's create(inOutSpec) to create the head file,
   /// and to createSplitFile(inOutSpec, options) to create the body file.
-  virtual void addSplitHead(FileSpec& inOutSpec) {}
+  virtual void addSplitHead(MAYBE_UNUSED FileSpec& inOutSpec) {}
 
   /// Tell if modifying files is supported by this FileHandler implementation.
   /// @return True if file modification and creation is supported.