tools/libxc: fix strncpy size

gcc-8 warns about possible truncation of trailing '\0'.
Final character is overridden by '\0' anyway, so don't bother to copy
it.

This fixes compile failure:

    xc_pm.c: In function 'xc_set_cpufreq_gov':
    xc_pm.c:308:5: error: 'strncpy' specified bound 16 equals destination size [-Werror=stringop-truncation]
         strncpy(scaling_governor, govname, CPUFREQ_NAME_LEN);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    cc1: all warnings being treated as errors

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Juergen Gross <jgross@suse.com>

tools/libxc/xc_pm.c

@@ -305,7 +305,7 @@ int xc_set_cpufreq_gov(xc_interface *xch, int cpuid, char *govname)
     sysctl.cmd = XEN_SYSCTL_pm_op;
     sysctl.u.pm_op.cmd = SET_CPUFREQ_GOV;
     sysctl.u.pm_op.cpuid = cpuid;
-    strncpy(scaling_governor, govname, CPUFREQ_NAME_LEN);
+    strncpy(scaling_governor, govname, CPUFREQ_NAME_LEN - 1);
     scaling_governor[CPUFREQ_NAME_LEN - 1] = '\0';
 
     return xc_sysctl(xch, &sysctl);