kernelland_emu: avoid dprintf conflict from stdio

pretending to be in kernel mode

Change-Id: Ib396f9aae0b6d8ebff0edc332a583c66c59f1067
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3283
Reviewed-by: Adrien Destugues <[email protected]>

src/tests/add-ons/kernel/kernelland_emu/debug.cpp

@@ -8,6 +8,7 @@
  */
 
 #include <signal.h>
+#define _KERNEL_MODE
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>