Fix build failure on Debian/Ubuntu

http://hydra.nixos.org/build/53537463

src/libutil/logging.cc

@@ -76,7 +76,7 @@ Logger * makeDefaultLogger()
     return new SimpleLogger();
 }
 
-std::atomic<uint64_t> Activity::nextId{(uint64_t) getpid() << 32};
+std::atomic<uint64_t> nextId{(uint64_t) getpid() << 32};
 
 Activity::Activity() : id(nextId++) { };
 

src/libutil/logging.hh

@@ -15,7 +15,6 @@ typedef enum {
 
 class Activity
 {
-    static std::atomic<uint64_t> nextId;
 public:
     typedef uint64_t t;
     const t id;

src/nix/progress-bar.cc

@@ -24,6 +24,7 @@ private:
         std::string uri;
         uint64_t current = 0;
         uint64_t expected = 0;
+        DownloadInfo(const std::string & uri) : uri(uri) { }
     };
 
     struct State