update devel/protobuf to 3.6.1

This is a very small update addressing the following:

C++ Introduced workaround for Windows issue with std::atomic and
std::once_flag initialization (#4777, #4773)

PHP Added compatibility with PHP 7.3 (#4898)

Ruby Fixed Ruby crash involving Any encoding (#4718)

Of these only the Ruby thing seems to be of direct relevance to us.

I build tested all consumers on amd64 and ran the test suites of the
libraries with no visible change to before the update.

For the record, audio/clementine and x11/kde4/marble error on running
'make test'.

Index: Makefile
===================================================================
RCS file: /var/cvs/ports/devel/protobuf/Makefile,v
retrieving revision 1.20
diff -u -p -r1.20 Makefile
--- Makefile 10 Jul 2018 10:47:42 -0000 1.20
+++ Makefile 2 Aug 2018 06:56:24 -0000
@@ -2,7 +2,7 @@

COMMENT = c++ protocol buffers

-V = 3.6.0
+V = 3.6.1
DISTNAME = protobuf-cpp-$V
PKGNAME = protobuf-$V

Index: distinfo
===================================================================
RCS file: /var/cvs/ports/devel/protobuf/distinfo,v
retrieving revision 1.4
diff -u -p -r1.4 distinfo
--- distinfo 10 Jul 2018 10:47:42 -0000 1.4
+++ distinfo 2 Aug 2018 06:57:03 -0000
@@ -1,2 +1,2 @@
-SHA256 (protobuf-cpp-3.6.0.tar.gz) = w8qwVZZNVU5P2FBn/j6etFyZFc689Tfpf6+qJFN2vOE=
-SIZE (protobuf-cpp-3.6.0.tar.gz) = 4454101
+SHA256 (protobuf-cpp-3.6.1.tar.gz) = s3MuRxqbt5UPCQ/QRX69JTapugiRt/N4WRnGVP4qJSk=
+SIZE (protobuf-cpp-3.6.1.tar.gz) = 4450975