python-protobuf: update dependencies

Since version 3.0.0 protobuf is compatible with Python3, remove
Python2-only dependency.

It depends on python-six now, add this missing dependency.
Signed-off-by: Alexey Roslyakov <alexey.roslyakov@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/python-protobuf/Config.in

 config BR2_PACKAGE_PYTHON_PROTOBUF
 	bool "python-protobuf"
-	depends on BR2_PACKAGE_PYTHON
 	# host-protobuf only builds on certain architectures
 	depends on BR2_HOSTARCH = "x86_64" || BR2_HOSTARCH = "x86"
 	depends on BR2_HOST_GCC_AT_LEAST_4_5 # host-protobuf
+	select BR2_PACKAGE_PYTHON_SIX # runtime
 	help
 	  Python implementation of the Google Protocol Buffers.