ship the bridges spec file in the tarball too

doc/spec/Makefile.am

@@ -1,5 +1,5 @@
 
 EXTRA_DIST = tor-spec.txt rend-spec.txt control-spec.txt     \
              dir-spec.txt socks-extensions.txt path-spec.txt \
-             version-spec.txt address-spec.txt
+             version-spec.txt address-spec.txt bridges-spec.txt