BUILD: debug: fix build warning by consuming the write() result

BUILD: debug: fix build warning by consuming the write() result

When writing commit a8459b28c ("MINOR: debug: create
ha_backtrace_to_stderr() to dump an instant backtrace") I just forgot
that some distros are a bit extremist about the syscall return values.

  src/debug.c: In function `ha_backtrace_to_stderr':
  src/debug.c:147:3: error: ignoring return value of `write', declared with attribute warn_unused_result [-Werror=unused-result]
     write(2, b.area, b.data);
     ^~~~~~~~~~~~~~~~~~~~~~~~
    CC      src/h1_htx.o

Let's apply the usual tricks to shut them up. No backport is needed.