[lld] Add missing header guard (NFC)

GitOrigin-RevId: 062d4ddd22c291bb1c0459bf8b8a5c4ee7d89a78
diff --git a/include/lld/Common/TargetOptionsCommandFlags.h b/include/lld/Common/TargetOptionsCommandFlags.h
index 422bb63..04428b5 100644
--- a/include/lld/Common/TargetOptionsCommandFlags.h
+++ b/include/lld/Common/TargetOptionsCommandFlags.h
@@ -10,6 +10,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLD_COMMON_TARGETOPTIONSCOMMANDFLAGS_H
+#define LLD_COMMON_TARGETOPTIONSCOMMANDFLAGS_H
+
 #include "llvm/ADT/Optional.h"
 #include "llvm/Support/CodeGen.h"
 #include "llvm/Target/TargetOptions.h"
@@ -21,3 +24,5 @@
 std::string getCPUStr();
 std::vector<std::string> getMAttrs();
 }
+
+#endif