gdbserver: Clear .deps on clean

gdbserver: Clear .deps on clean

In some situations, the dependency tracking files in .deps can refer to
source files that were removed or renamed, leading to errors like:

  make: *** No rule to make target `version.c', needed by `version.o'. Stop.

This patch makes the clean target clear the .deps directory, which gives
the user a chance to recover from the error wihtout knowing about the
internals of the build system.

It is already done for GDB.  See here for more details:

  https://sourceware.org/ml/gdb-patches/2009-03/msg00000.html

gdb/gdbserver/ChangeLog:

	* Makefile.in (clean): Clear .deps.