Avoid shadowing 'free' in X509_LOOKUP_met_set_free

gcc 4.6 (arguably erroneously) warns about our use of 'free' as
the name of a function parameter, when --strict-warnings is enabled:

crypto/x509/x509_meth.c: In function 'X509_LOOKUP_meth_set_free':
crypto/x509/x509_meth.c:61:12: error: declaration of 'free' shadows a global declaration [-Werror=shadow]
cc1: all warnings being treated as errors
make[1]: *** [crypto/x509/x509_meth.o] Error 1

(gcc 4.8 is fine with this code, as are newer compilers.)

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6991)