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>

lib/igt_debugfs.c

@@ -27,6 +27,7 @@
 #include <sys/mount.h>
 #include <errno.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <fcntl.h>
 #include <unistd.h>