prometheus-cpp: propagate curl and zlib

These are needed when building a downstream package that uses CMake
to find prometheus-cpp as a dependency.

pkgs/by-name/pr/prometheus-cpp/package.nix

···
     pkg-config
   ];
   buildInputs = [
-    curl
     gbenchmark
     gtest
+  ];
+  propagatedBuildInputs = [
+    civetweb
+    curl
     zlib
   ];
-  propagatedBuildInputs = [ civetweb ];
   strictDeps = true;
 
   cmakeFlags = [