remove "using namespace std" from headers

It's nasty, and actually breaks on Solaris, where if.h fails to
build due to:

  struct map *if_memmap;

src/blockchain_utilities/bootstrap_file.h

@@ -57,7 +57,7 @@ class BootstrapFile
 public:
 
   uint64_t count_bytes(std::ifstream& import_file, uint64_t blocks, uint64_t& h, bool& quit);
-  uint64_t count_blocks(const std::string& dir_path, streampos& start_pos, uint64_t& seek_height);
+  uint64_t count_blocks(const std::string& dir_path, std::streampos& start_pos, uint64_t& seek_height);
   uint64_t count_blocks(const std::string& dir_path);
   uint64_t seek_to_first_chunk(std::ifstream& import_file);