lib: unnecessary header removal for drmtest.h, part 2

I've left unistd.h in it - it's not strictly required but most users
of drmtest.h want it for the open helpers, and then you kinda need to
close that file descriptor again ...

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

tests/pm_rps.c

@@ -34,6 +34,7 @@
 #include <getopt.h>
 #include <fcntl.h>
 #include <signal.h>
+#include <errno.h>
 
 #include "drmtest.h"
 #include "intel_gpu_tools.h"