Fix buffer size

../gntwm.c:819:47: warning: ‘__builtin___snprintf_chk’ output may be
truncated before the last format character [-Wformat-truncation=]
/usr/include/x86_64-linux-gnu/bits/stdio2.h:67:10: note:
‘__builtin___snprintf_chk’ output between 8 and 13 bytes into a
destination of size 12

Testing Done:
With this change, the compiler warning is eliminated.

Reviewed at https://reviews.imfreedom.org/r/475/