Add *.d.tmp files to .gitignore

These are temporary files generated by the build process that should not
be checked in.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11122)

.gitignore

@@ -220,6 +220,7 @@ Makefile.save
 *.bak
 cscope.*
 *.d
+*.d.tmp
 pod2htmd.tmp
 
 # Windows manifest files