libfetchers: Improve compile times with PCH

(Before)

**** Time summary:
Compilation (17 times):
  Parsing (frontend):           72.3 s
  Codegen & opts (backend):     47.8 s

(After)

Compilation (19 times):
  Parsing (frontend):           27.5 s
  Codegen & opts (backend):     43.8 s

src/libfetchers/git-lfs-fetch.cc

@@ -292,7 +292,7 @@ void Fetch::fetch(
             auto authIt = headerIt->find("Authorization");
             if (authIt == headerIt->end())
                 return std::nullopt;
-            return *authIt;
+            return std::string(*authIt);
         }();
         const uint64_t size = obj.at("size");
         sizeCallback(size);

src/libfetchers/meson.build

@@ -66,6 +66,7 @@ this_library = library(
   link_args : linker_export_flags,
   prelink : true, # For C++ static initializers
   install : true,
+  cpp_pch : do_pch ? [ 'pch/precompiled-headers.hh' ] : [],
 )
 
 install_headers(headers, subdir : 'nix/fetchers', preserve_path : true)

src/libfetchers/pch/precompiled-headers.hh

@@ -0,0 +1,3 @@
+#include "nix/fetchers/fetchers.hh"
+#include "nix/store/store-api.hh"
+#include "nix/util/json-utils.hh"