Add test/bio_prefix_text to .gitignore

A new test binary was added as part of
51a7c4b
(from #10531 ).

This commit adds said binary to .gitignore to avoid cluttering of the
worktree.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from #10768)

.gitignore

@@ -97,6 +97,7 @@ doc/man1/openssl.pod
 /test/ossl_shim/ossl_shim
 /test/rsa_complex
 /test/confdump
+/test/bio_prefix_text
 # Other generated files in test/
 /test/provider_internal_test.conf
 /test/fipsinstall.conf