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_core.h

@@ -32,8 +32,10 @@
 
 #include <setjmp.h>
 #include <stdbool.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
+#include <stdarg.h>
 
 bool __igt_fixture(void);
 void __igt_fixture_complete(void);