tools: .gitignore: add prelink-riscv

Ignore tools/prelink-riscv.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/tools/.gitignore b/tools/.gitignore
index c8cdaef..e5ede22 100644
--- a/tools/.gitignore
+++ b/tools/.gitignore
@@ -24,6 +24,7 @@
 /mksunxiboot
 /mxsboot
 /ncb
+/prelink-riscv
 /proftool
 /relocate-rela
 /sunxi-spl-image-builder