qemu: MULTI_CONSOLE_API=0 causes build error

Add crash_console_init declaration to console.h
Only enable MULTI_CONSOLE_API for AArch64

Fixes ARM-software/tf-issues#571

Signed-off-by: Michalis Pappas <mpappas@fastmail.fm>
diff --git a/plat/qemu/qemu_console.c b/plat/qemu/qemu_console.c
index 9c02957..1cf84aa 100644
--- a/plat/qemu/qemu_console.c
+++ b/plat/qemu/qemu_console.c
@@ -7,8 +7,10 @@
 #include <pl011.h>
 #include <platform_def.h>
 
+#if MULTI_CONSOLE_API
 static console_pl011_t console;
 static console_pl011_t crash_console;
+#endif /* MULTI_CONSOLE_API */
 
 void qemu_console_init(void)
 {