gdbserver: Fix build on MIPS

Commit 3470a0e144df6c01f8479fa649f43aa907936e7e inadvertently broke
the build on MIPS because it's passing a non-existent "pid" argument
to "proc->for_each_thread".  This commit fixes the problem by removing
the argument from the call.

Signed-off-by: Sergio Durigan Junior <sergiodj@sergiodj.net>