Fix build failure with -Werror=format-security.

[sebastic]

The Debian package build for NRPE 3.1.0 failed due to the hardening buildflags:

./utils.c: In function 'logit':
./utils.c:530:4: error: format not a string literal and no format arguments [-Werror=format-security]
    syslog(priority, buffer);
    ^~~~~~

As documented in syslog(3) the second argument should be a format string:

void syslog(int priority, const char *format, ...);
...
syslog() generates a log message, which will be distributed by syslogd(8). The priority argument is formed by ORing the facility and the level values (explained below). The remaining arguments are a format, as in printf(3) and any arguments required by the format, except that the two character sequence %m will be replaced by the error message string strerror(errno). A trailing newline may be added if needed.

[smooge]

Thank you for putting this one in place. I was about to put in a PR for the same fix. Sorry for not catching it sooner.

[jfrickson]

Merged. Thanks!