glib/tests: Clean up inclusion of unistd.h

Include unistd.h only when G_OS_UNIX is defined (or when G_OS_WIN32 is not
defined).  This will avoid including unistd.h unconditionally and/or
unecessarily, which may cause problems in certain scenarios, such as when
building the tests on Visual C++, which does not come with a unistd.h and
MinGW, where unistd.h is essentially a wrapper for io.h and process.h.

https://bugzilla.gnome.org/show_bug.cgi?id=711047

glib/tests/thread.c

@@ -26,17 +26,14 @@
 #include <sys/time.h>
 #endif
 #include <sys/types.h>
-#ifdef HAVE_UNISTD_H
-#include <unistd.h>
-#endif
 #ifdef HAVE_SYS_PRCTL_H
 #include <sys/prctl.h>
 #endif
 
-
 #include <glib.h>
 
 #ifndef G_OS_WIN32
+#include <unistd.h>
 #include <sys/resource.h>
 #endif