silence unused variable warnings when using OPENSSL_clear_free

e.g. here: https://github.com/nodejs/node/blob/adbe3b837e8a2285238ec0fcba89c20882eb4cdb/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

diff --git a/crypto/mem.c b/crypto/mem.c
index a06061b..6af532e 100644
--- a/crypto/mem.c
+++ b/crypto/mem.c
@@ -153,6 +153,10 @@
 #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

diff --git a/include/openssl/mem.h b/include/openssl/mem.h
index 4e1c2ca..2e25f52 100644
--- a/include/openssl/mem.h
+++ b/include/openssl/mem.h
@@ -138,7 +138,7 @@
 
 // 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)