package/opencv3: fix build with protobuf and gcc < 6

Fix the folloing build failure with protobuf (enabled since commit
31c68a44) and gcc 5.3.0:

[ 53%] Building CXX object modules/dnn/CMakeFiles/opencv_dnn.dir/opencv-caffe.pb.cc.o
In file included from /home/peko/autobuild/instance-1/output-1/per-package/opencv3/host/opt/ext-toolchain/mips64el-buildroot-linux-uclibc/include/c++/5.5.0/atomic:38:0,
                 from /home/peko/autobuild/instance-1/output-1/per-package/opencv3/host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/google/protobuf/io/coded_stream.h:115,
                 from /home/peko/autobuild/instance-1/output-1/build/opencv3-3.4.12/buildroot-build/modules/dnn/opencv-caffe.pb.h:23,
                 from /home/peko/autobuild/instance-1/output-1/build/opencv3-3.4.12/buildroot-build/modules/dnn/opencv-caffe.pb.cc:4:
/home/peko/autobuild/instance-1/output-1/per-package/opencv3/host/opt/ext-toolchain/mips64el-buildroot-linux-uclibc/include/c++/5.5.0/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
 #error This file requires compiler and library support \
  ^

Fixes:
 - http://autobuild.buildroot.org/results/7caf175af039054a032b8f63b458b3940d9ec0f3



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>