Fix -Wformat-truncation warning

planemask.c: In function ‘create_planemask_choice’:
planemask.c:109:43: warning: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 3 [-Wformat-truncation=]
     snprintf(name, sizeof name, "planemask%d",i);
                                           ^~
planemask.c:109:33: note: directive argument in the range [0, 2147483647]
     snprintf(name, sizeof name, "planemask%d",i);
                                 ^~~~~~~~~~~~~
planemask.c:109:5: note: ‘snprintf’ output between 11 and 20 bytes into a destination of size 12
     snprintf(name, sizeof name, "planemask%d",i);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
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