Add missing external. (#53)

Signed-off-by: John Plevyak <jplevyak@gmail.com>

Author: jplevyak

proxy_wasm_intrinsics.js

@@ -30,6 +30,7 @@ mergeInto(LibraryManager.library, {
   proxy_add_header_map_value: function() {},
   proxy_get_header_map_value: function() {},
   proxy_get_header_map_pairs: function() {},
+  proxy_set_header_map_pairs: function() {},
   proxy_get_header_map_size: function() {},
   proxy_get_shared_data: function() {},
   proxy_set_shared_data: function() {},