Fix recent breakage of win32 build


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18876 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/System/Win32/MappedFile.cpp b/lib/System/Win32/MappedFile.cpp
index d81ef6a..982a3b2 100644
--- a/lib/System/Win32/MappedFile.cpp
+++ b/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;
 }