Clarify dual license status of subprocess.py file.

The author of the file we stole from Python 2.4 distribution, Peter
Astrand <astrand@lysator.liu.se>, OK'ed to add this at the end of the
licensing terms section of the file:

    Use of this file within git is permitted under GPLv2.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
(cherry picked from 089f20dc commit)

compat/subprocess.py

@@ -24,6 +24,9 @@
 # OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
 # NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION
 # WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+#
+# Use of this file within git is permitted under GPLv2.
+#
 
 r"""subprocess - Subprocesses with accessible I/O streams