spl: Disable printf if not required

Now we have a guard for printf, disable it in the build if it's not
selected.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>

lib/vsprintf.c

@@ -783,6 +783,7 @@ int sprintf(char *buf, const char *fmt, ...)
 	return i;
 }
 
+#if CONFIG_IS_ENABLED(PRINTF)
 int printf(const char *fmt, ...)
 {
 	va_list args;
@@ -824,7 +825,7 @@ int vprintf(const char *fmt, va_list args)
 	puts(printbuffer);
 	return i;
 }
-
+#endif
 
 void __assert_fail(const char *assertion, const char *file, unsigned line,
 		   const char *function)