Core/SecretString: Use `memset_s` instead of `explicit_bzero` on MacOS

MacOS doesn't have `explicit_bzero`, so this was causing errors when
compiling LibCore on the host.

Userland/Libraries/LibCore/SecretString.cpp

@@ -1,10 +1,15 @@
 /*
  * Copyright (c) 2021, Brian Gianforcaro <bgianf@serenityos.org>
+ * Copyright (c) 2021, Mustafa Quraish <mustafa@cs.toronto.edu>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Platform.h>
 #include <LibCore/SecretString.h>
+#if defined(AK_OS_MACOS)
+#    define __STDC_WANT_LIB_EXT1__ 1
+#endif
 #include <string.h>
 
 namespace Core {
@@ -14,7 +19,11 @@ SecretString SecretString::take_ownership(char*& cstring, size_t length)
     auto buffer = ByteBuffer::copy(cstring, length);
     VERIFY(buffer.has_value());
 
+#if defined(AK_OS_MACOS)
+    memset_s(cstring, length, 0, length);
+#else
     explicit_bzero(cstring, length);
+#endif
     free(cstring);
 
     return SecretString(buffer.release_value());
@@ -32,10 +41,14 @@ SecretString::SecretString(ByteBuffer&& buffer)
 
 SecretString::~SecretString()
 {
+    // Note: We use explicit_bzero to avoid the zeroing from being optimized out by the compiler,
+    // which is possible if memset was to be used here.
     if (!m_secure_buffer.is_empty()) {
-        // Note: We use explicit_bzero to avoid the zeroing from being optimized out by the compiler,
-        // which is possible if memset was to be used here.
+#if defined(AK_OS_MACOS)
+        memset_s(m_secure_buffer.data(), m_secure_buffer.size(), 0, m_secure_buffer.size());
+#else
         explicit_bzero(m_secure_buffer.data(), m_secure_buffer.capacity());
+#endif
     }
 }