Make lexer-helpers.hh internal to fix a clang-tidy error

(cherry picked from commit 8be24f58f2)

src/libexpr/lexer-helpers.hh

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <cstddef>
+
+// inluding the generated headers twice leads to errors
+#ifndef BISON_HEADER
+#  include "lexer-tab.hh"
+#  include "parser-tab.hh"
+#endif
+
+namespace nix::lexer::internal {
+
+void initLoc(YYLTYPE * loc);
+
+void adjustLoc(yyscan_t yyscanner, YYLTYPE * loc, const char * s, size_t len);
+
+} // namespace nix::lexer