tools: prelink-riscv: Unmap the ELF image when done

The codes forget to call munmap() to unmap the ELF image that was
mapped by previous mmap().

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/tools/prelink-riscv.c b/tools/prelink-riscv.c
index b046794..43d6412 100644
--- a/tools/prelink-riscv.c
+++ b/tools/prelink-riscv.c
@@ -118,5 +118,7 @@
 			prelink_le32(data);
 	}
 
+	munmap(data, st.st_size);
+
 	return 0;
 }