[gdbserver] Update a couple Makefile rules.

[gdbserver] Update a couple Makefile rules.

These two are still written in the pre-auto-dependency-tracking style.
They probably were written before that, and committed afterwards
without adjustment.  An easy oversight to make.

gdb/gdbserver/
2013-09-06  Pedro Alves  <palves@redhat.com>

	* Makefile.in (linux-btrace.o, mips-linux-watch.o): Remove
	explicit header dependencies and use $COMPILE/$POSTCOMPILE.