common/Preforker: fix warning

common/Preforker: fix warning

common/Preforker.h: In member function ‘int Preforker::signal_exit(int)’:
warning: common/Preforker.h:82:45: ignoring return value of ‘ssize_t safe_write(int, const void*, size_t)’, declared with attribute warn_unused_result [-Wunused-result]

This is harder than it should be to fix.  :(
  http://stackoverflow.com/questions/3614691/casting-to-void-doesnt-remove-warn-unused-result-error

Whatever, I guess we can do something useful with this return value.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>