File: Make GetSize() a const member function

Source/Core/Common/File.cpp

@@ -85,7 +85,7 @@ void IOFile::SetHandle(std::FILE* file)
   m_file = file;
 }
 
-u64 IOFile::GetSize()
+u64 IOFile::GetSize() const
 {
   if (IsOpen())
     return File::GetSize(m_file);

Source/Core/Common/File.h

@@ -74,7 +74,7 @@ public:
 
   bool Seek(s64 off, int origin);
   u64 Tell() const;
-  u64 GetSize();
+  u64 GetSize() const;
   bool Resize(u64 size);
   bool Flush();