[windows] Fix accidental change of %u -> %d in SuspendThread warning.

While enhancing the warning printed in when SuspendThread fails,
I accidently changed the format used to print the error code
from %u to %d. This patch reverts it back.

gdb/ChangeLog:

        * windows-nat.c (thread_rec): Revert format used to print
        error code returned by SuspendThread from %d back to %u.
windows-nat.c:thread_rec: Add missing empty line after var declaration.

gdb/ChangeLog:

        * window-nat.c (thread_rec): Add missing empty line after
        local variable declaration.