fix missing includes in various headers

src/libutil/callback.hh

@@ -1,6 +1,7 @@
 #pragma once
 ///@file
 
+#include <cassert>
 #include <future>
 #include <functional>
 

src/libutil/config-impl.hh

@@ -13,6 +13,7 @@
  */
 
 #include "config.hh"
+#include "args.hh"
 
 namespace nix {
 

src/libutil/regex-combinators.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include <string_view>
+#include <string>
 
 namespace nix::regex {
 

src/libutil/unix/signals-impl.hh

@@ -14,6 +14,7 @@
 #include "error.hh"
 #include "logging.hh"
 #include "ansicolor.hh"
+#include "signals.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>