build: clang: Fix -Wfortify-source
xfce4-cpufreq-linux-procfs.cc:123:17: error: 'sscanf' may overflow; destination buffer in argument 5 has size 20, but the corresponding specifier may require size 21 [-Werror,-Wfortify-source] 123 | gov);
xfce4-cpufreq-linux-procfs.cc:123:17: error: 'sscanf' may overflow; destination buffer in argument 5 has size 20, but the corresponding specifier may require size 21 [-Werror,-Wfortify-source] 123 | gov);