cgi-io: fix stray semicolon

cgi-io: fix stray semicolon

Remove an improperly placed semicolon in order to solve the following
compiler error:

.../main.c:144:3: error: this 'if' clause does not guard... [-Werror=misleading-indentation]
    if (execl("/bin/busybox", "/bin/busybox", "md5sum", file, NULL));
    ^~
    .../main.c:145:4: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
    return NULL;
    ^~~~~~
    cc1: all warnings being treated as errors

Fixes #4723.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>