Add missing exports.

Reported by Taylan Ulrich Bayırlı/Kammer <taylanbayirli@gmail.com>
at <https://lists.gnu.org/archive/html/guix-devel/2016-09/msg01686.html>.

* gnu/system/file-systems.scm (%tty-gid): Export.
* guix/build-system/python.scm (default-python, default-python2):
Export.

gnu/system/file-systems.scm

@@ -47,6 +47,7 @@
             %binary-format-file-system
             %shared-memory-file-system
             %pseudo-terminal-file-system
+            %tty-gid
             %immutable-store
             %control-groups
             %elogind-file-systems

guix/build-system/python.scm

@@ -33,6 +33,8 @@
   #:export (%python-build-system-modules
             package-with-python2
             strip-python2-variant
+            default-python
+            default-python2
             python-build
             python-build-system
             pypi-uri))