scons: Add PROTOC from the environment
scons: Add PROTOC from the environment

This patch adds PROTOC to the build environment.

haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5