libpayload: Fix compilation on ARM with GDB enabled

Without this, gdb_enter() is not defined.

Change-Id: I067dce371ee817d6ac77387fcbe42a9a7deb6438
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10755
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)

payloads/libpayload/arch/arm/virtual.c

@@ -27,6 +27,7 @@
  * SUCH DAMAGE.
  */
 
+#include <libpayload.h>
 #include <assert.h>
 #include <die.h>
 #include <stdlib.h>