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: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6991)

crypto/x509/x509_meth.c

@@ -58,9 +58,9 @@ int (*X509_LOOKUP_meth_get_new_item(const X509_LOOKUP_METHOD* method))
 
 int X509_LOOKUP_meth_set_free(
     X509_LOOKUP_METHOD *method,
-    void (*free) (X509_LOOKUP *ctx))
+    void (*free_fn) (X509_LOOKUP *ctx))
 {
-    method->free = free;
+    method->free = free_fn;
     return 1;
 }
 

include/openssl/x509_vfy.h

@@ -401,7 +401,7 @@ int (*X509_LOOKUP_meth_get_new_item(const X509_LOOKUP_METHOD* method))
     (X509_LOOKUP *ctx);
 
 int X509_LOOKUP_meth_set_free(X509_LOOKUP_METHOD *method,
-                              void (*free) (X509_LOOKUP *ctx));
+                              void (*free_fn) (X509_LOOKUP *ctx));
 void (*X509_LOOKUP_meth_get_free(const X509_LOOKUP_METHOD* method))
     (X509_LOOKUP *ctx);