[klibc] fflush: add missing parenthesis

Noticed by current cppcheck, all other stdio code is clean.

Signed-off-by: maximilian attems <max@stro.at>
diff --git a/usr/klibc/stdio/fflush.c b/usr/klibc/stdio/fflush.c
index 434fc51..0970c68 100644
--- a/usr/klibc/stdio/fflush.c
+++ b/usr/klibc/stdio/fflush.c
@@ -18,7 +18,7 @@
 		return err;
 	}
 
-	if (!f->flags & _IO_FILE_FLAG_WRITE)
+	if (!(f->flags & _IO_FILE_FLAG_WRITE))
 		return 0;
 
 	p = f->buf;