fix(gcc): Fix -Wstringop-truncation warning
fix(gcc): Fix -Wstringop-truncation warning

As mentioned in [1], gcc >=8 will complain, if strncpy truncates the
source string or gcc can prove there is no NUL terminating byte.

The header_t.magic field is a non-NUL terminated 6 byte string, so we
use memcpy here

[1] https://github.com/jaagr/polybar/issues/1215

jaagr/i3ipcpp,jaagr/i3ipcpp,jaagr/i3ipcpp,jaagr/i3ipcpp