mkvers: fix potential buffer overflow

mkvers: fix potential buffer overflow

The space allocated for outputFileBuf is only 2 bytes larger than
sizeof(VERS_FILE). But we add potentially 4 extra bytes like
".txt" or ".xml". Just allocate enough space for all file suffices.

Change-Id: Ic0f97590be208deaf9c4a5c25e21056ea9d2cd6f
Reviewed-on: https://gerrit.openafs.org/12657
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>