libexpr/gc-alloc: fix compilation with !HAVE_BOEHMGC

Fixes: 72ee25b402
Change-Id: Ib59386af1415a8ed4b53af24ec22a4ffa5e5877d

src/libexpr/gc-alloc.hh

@@ -10,6 +10,8 @@
 #include <string_view>
 #include <vector>
 
+#include "checked-arithmetic.hh"
+
 #if HAVE_BOEHMGC
 #include <functional> // std::less
 #include <utility> // std::pair
@@ -18,8 +20,6 @@
 #include <gc/gc_allocator.h>
 #include <gc/gc_cpp.h>
 
-#include "checked-arithmetic.hh"
-
 /// calloc, transparently GC-enabled.
 #define LIX_GC_CALLOC(size) GC_MALLOC(size)