gcc kernel build fail @ SVN r256730

[Michael Butler]

-current kernel compiled with gcc fails with:

cc1: warnings being treated as errors
/usr/src/sys/geom/label/g_label.c: In function 'g_label_resize':
/usr/src/sys/geom/label/g_label.c:135: warning: null format string
[-Wformat]
*** [g_label.o] Error code 1