do not remove links found in .git directory

Some setups use links inside .git directory and make clean should not
remove them to avoid breaking git meta-information.

Signed-off-by: Cristian Stoica <cristian.stoica@nxp.com>

CLA: none; trivial
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1078)