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/core_getclient.c

@@ -25,12 +25,12 @@
  *
  */
 
+#include "igt.h"
 #include <limits.h>
 #include <sys/ioctl.h>
 #include <stdlib.h>
 #include <errno.h>
 
-#include "drmtest.h"
 
 IGT_TEST_DESCRIPTION("Tests the DRM_IOCTL_GET_CLIENT ioctl.");