Use '$enable_unittest' instead of '$development' on gdbserver/configure.srv...

Use '$enable_unittest' instead of '$development' on gdbserver/configure.srv (for 'aarch64*-*-linux*' case)

On commit 8ecfd7bd ("Add parameter to
allow enabling/disabling selftests via configure") it seems that I
forgot to use the proper '$enable_unittest' variable when checking to
see whether to add selftest-related objects to 'srv_regobj'.  This
causes a build failure on Aarch64 when 'development=false' (which is
the case for the 8.3 branch) and 'enable_unittest=true'.

This patch fixes the problem by using '$enable_unittest' instead of
'$development' when performing the check.  As a reminder, it's
important to notice that '$enable_unittest's default value (i.e., when
the option '--enable-unit-tests' is not passed to configure) is the
same as '$development', so this patch doesn't affect the current
build.

I'd like to install this patch both on master and on the 8.3 branch.

OK?

gdb/gdbserver/ChangeLog:
2019-03-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* configure.srv: Use '$enable_unittest' instead of '$development'
	when checking whether to fill 'srv_regobj' on 'aarch64*-*-linux*'
	case.