Add missing header guards

libsolc/libsolc.h

@@ -20,6 +20,8 @@
  * Public compiler API.
  */
 
+#pragma once
+
 #include <stdbool.h>
 
 #ifdef __cplusplus

libsolidity/codegen/ExpressionCompiler.h

@@ -21,6 +21,8 @@
  * Solidity AST to EVM bytecode compiler for expressions.
  */
 
+#pragma once
+
 #include <functional>
 #include <memory>
 #include <boost/noncopyable.hpp>

libyul/backends/evm/EVMCodeTransform.h

@@ -18,6 +18,8 @@
  * Common code generator for translating Yul / inline assembly to EVM and EVM1.5.
  */
 
+#pragma once
+
 #include <libyul/backends/evm/EVMAssembly.h>
 
 #include <libyul/backends/evm/EVMDialect.h>

libyul/backends/evm/EVMObjectCompiler.h

@@ -18,6 +18,7 @@
  * Compiler that transforms Yul Objects to EVM bytecode objects.
  */
 
+#pragma once
 
 namespace yul
 {