gtestutils: Add missing include
gtestutils: Add missing include

memcmp() is used, which is declared in string.h. Include that.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=791532

endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib