Fix recent breakage of win32 build

llvm-svn: 18876
diff --git a/llvm/lib/System/Win32/MappedFile.cpp b/llvm/lib/System/Win32/MappedFile.cpp
index d81ef6a..982a3b2 100644
--- a/llvm/lib/System/Win32/MappedFile.cpp
+++ b/llvm/lib/System/Win32/MappedFile.cpp
@@ -97,7 +97,7 @@
   return base_;
 }
 
-size_t MappedFile::size() {
+size_t MappedFile::size() const {
   assert(info_ && "MappedFile not initialized");
   return info_->size;
 }