gio/tests: fix race when generating code

There is a race condition in the makefile that can result in build failures like this in parallel builds:

| ./gdbus-test-codegen-generated.h:7:0: error: unterminated #ifndef
|  #ifndef __GDBUS_TEST_CODEGEN_GENERATED_H__

This is because a rule like this:

x.c x.h: prerequisites
	@commands

doesn't consider x.c and x.h together. Instead, it expands to two rules, one to
generate x.c and one to generate x.h, which happen to run the same commands.  In
the worst case they execute in parallel, overwriting each other's output.

Signed-off-by: Ross Burton <ross.burton@intel.com>

https://bugzilla.gnome.org/show_bug.cgi?id=723616