Analyse following RPM build log snippet. Describe contents accurately, without speculation or suggestions for resolution.            
Your analysis must be as concise as possible, while keeping relevant information intact.
Snippet: 
src/spss/readstat_por_write.c: In function 'por_emit_header':
src/spss/readstat_por_write.c:206:5: error: 'strncpy' output may be truncated copying 20 bytes from a string of length 256 [-Werror=stringop-truncation]
  206 |     strncpy(vanity[1] + 20, writer->file_label, 20);
      |     ^
cc1: all warnings being treated as errors
This snippet shows the GCC error which causes the build failure.