core -> extra for mac

Author: James Miller

src/etc/snapshot.py

@@ -34,7 +34,7 @@ def scrub(b):
               "lib/librustllvm.so"],
     "macos": ["bin/rustc",
               "lib/libstd-*.dylib",
-              "lib/libcore-*.dylib",
+              "lib/libextra-*.dylib",
               "lib/librustc-*.dylib",
               "lib/libsyntax-*.dylib",
               "lib/librustrt.dylib",