build: Enforce the commit to be GPG signed

Signed-off-by: David Goulet <dgoulet@torproject.org>

build.sh

@@ -136,5 +136,5 @@ do
 
     # Add file for staging area and commit.
     runcmd git add "$SIG_PATH"
-    runcmd git commit -s -m "$keyid: Signature for tor.git $VERSION"
+    runcmd git commit -S -s -m "$keyid: Signature for tor.git $VERSION"
 done