silence unused variable warnings when using OPENSSL_clear_free

e.g. here: adbe3b837e/src/node_crypto.cc (L3439)

Change-Id: I2d43a3439d6a56c8eee3636b3c1f5ba615b233ba
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/35144
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

crypto/mem.c

@@ -153,6 +153,10 @@ void OPENSSL_cleanse(void *ptr, size_t len) {
 #endif  // !OPENSSL_NO_ASM
 }
 
+void OPENSSL_clear_free(void *ptr, size_t unused) {
+  OPENSSL_free(ptr);
+}
+
 int CRYPTO_memcmp(const void *in_a, const void *in_b, size_t len) {
   const uint8_t *a = in_a;
   const uint8_t *b = in_b;

include/openssl/mem.h

@@ -138,7 +138,7 @@ OPENSSL_EXPORT int BIO_vsnprintf(char *buf, size_t n, const char *format,
 
 // OPENSSL_clear_free calls |OPENSSL_free|. BoringSSL automatically clears all
 // allocations on free, but we define |OPENSSL_clear_free| for compatibility.
-#define OPENSSL_clear_free(ptr, len) OPENSSL_free(ptr)
+OPENSSL_EXPORT void OPENSSL_clear_free(void *ptr, size_t len);
 
 
 #if defined(__cplusplus)