[gdb] Fix clang buildbreaker

[gdb] Fix clang buildbreaker

Building gdb with clang, I run into:
...
src/gdb/gdbserver/linux-low.c:6190:41: error: comparison of unsigned \
  expression < 0 is always false [-Werror,-Wtautological-compare]
          if (debug_write ("sigchld_handler\n",
                           sizeof ("sigchld_handler\n") - 1) < 0)
                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~
...

This regression is introduced by commit a7e559cc08 "gdbserver: Ensure all
debug output uses debug functions", which replaces calls to write with result
type ssize_t with calls to debug_write with result type size_t.

Fix this by making debug_write return ssize_t.

Build and reg-tested on x86_64-linux.

gdb/gdbserver/ChangeLog:

2019-06-19  Tom de Vries  <tdevries@suse.de>

	* debug.h (debug_write): Change return type to ssize_t.
	* debug.c (debug_write): Same.