cgsnapshot: Fix compiler truncation warning

cgsnapshot: Fix compiler truncation warning

Fix the following warning when running make distcheck

../../../../src/tools/cgsnapshot.c: In function ‘is_ctlr_on_list’:
../../../../src/tools/cgsnapshot.c:532:57: warning: ‘%s’ directive output may be truncated writing up to 409599 bytes into a region of size 4096 [-Wformat-truncation=]
532 |                 snprintf(controllers[i], FILENAME_MAX, "%s", tmp_controllers[i]);
    |                                                         ^~

Suggested-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>