Don't expose printf if we don't have a stdout.

The Arty A7 bootloader uses some of the RTOS headers but does not define
the UART in the normal way (and can't because it runs before the loader)
and so this is broken.

sdk/include/stdio.h

diff --git a/sdk/include/stdio.h b/sdk/include/stdio.h
index fd6b605..9448b5c 100644
--- a/sdk/include/stdio.h
+++ b/sdk/include/stdio.h
@@ -48,6 +48,7 @@
 	return ret;
 }
 
+#ifdef stdout
 static inline int printf(const char *format, ...)
 {
 	va_list ap;
@@ -57,6 +58,7 @@
 	va_end(ap);
 	return ret;
 }
+#endif
 
 int __cheri_libcall snprintf(char *str, size_t size, const char *format, ...);
 int __cheri_libcall vsnprintf(const char *str,