protobuf-c: fix host build on GCC4

prereq-build.mk says that GCC 4.8 and above is supported. GCC 4.8
does not default to gnu++11. This fixes compilation.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/protobuf-c/Makefile

@@ -48,6 +48,7 @@ endef
 
 CMAKE_HOST_OPTIONS += \
 	-DBUILD_SHARED_LIBS=ON \
+	-DCMAKE_CXX_STANDARD=11 \
 	-DCMAKE_SKIP_RPATH=OFF \
 	-DCMAKE_INSTALL_RPATH="${STAGING_DIR_HOSTPKG}/lib"