Use a forward declaration. NFC.

llvm-svn: 303363
diff --git a/lld/ELF/MapFile.cpp b/lld/ELF/MapFile.cpp
index 23c63e8..c3f4f5a 100644
--- a/lld/ELF/MapFile.cpp
+++ b/lld/ELF/MapFile.cpp
@@ -21,6 +21,7 @@
 
 #include "MapFile.h"
 #include "InputFiles.h"
+#include "OutputSections.h"
 #include "Strings.h"
 #include "SymbolTable.h"
 #include "Threads.h"