Fix the build after bfd_get_signed_8 change

A recent commit changed bfd_get_signed_8 to extend the result to a
bfd_signed_vma.  This caused a compiler error in one spot in my
--enable-targets=all gdb build, where the result of bfd_get_signed_8
was passed to printf.

This patch fixes the build.  Tested by rebuilding.

gdb/ChangeLog
2019-12-11  Tom Tromey  <tromey@adacore.com>

	* fbsd-tdep.c (fbsd_core_info_proc_status): Cast result of
	bfd_get_signed_8.

Change-Id: Ic015f5fd3d88da6b5da8f7b4e1d11d5c981333db