add win32-specific explicit_bzero implementation

crypto/compat/explicit_bzero_win.c

@@ -0,0 +1,13 @@
+/*
+ * Public domain.
+ * Win32 explicit_bzero compatibility shim.
+ */
+
+#include <windows.h>
+#include <string.h>
+
+void
+explicit_bzero(void *buf, size_t len)
+{
+	SecureZeroMemory(buf, len);
+}