lib: add a single include header

Add a header that includes all the headers for the library. This allows
reorganisation of the library without affecting programs using it and
also simplifies the headers that need to be included to use the library.

Signed-off-by: Thomas Wood <thomas.wood@intel.com>

tests/kms_legacy_colorkey.c

@@ -21,11 +21,9 @@
  * IN THE SOFTWARE.
  */
 
+#include "igt.h"
 #include <errno.h>
 
-#include "drmtest.h"
-#include "igt_kms.h"
-#include "igt_aux.h"
 
 IGT_TEST_DESCRIPTION("Check that the legacy set colorkey ioctl only works on sprite planes.");