Change isc_buffer_reallocate() into a static functions as it is not used...

Change isc_buffer_reallocate() into a static functions as it is not used outside of isc_buffer_reserve()

(cherry picked from commit 20faf465)
13 jobs for 443-isc_buffer_printf-fixes-v9_12 in 9 minutes and 25 seconds (queued for 7 seconds)