compat: document S_I* defines when building for Windows

src/compat/compat.h

@@ -60,6 +60,8 @@ typedef unsigned int SOCKET;
 #endif
 #endif
 
+// Windows doesn't define S_IRUSR or S_IWUSR. We define both
+// here, with the same values as glibc (see stat.h).
 #ifdef WIN32
 #ifndef S_IRUSR
 #define S_IRUSR             0400

src/wallet/bdb.cpp

@@ -3,6 +3,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <compat/compat.h>
 #include <fs.h>
 #include <wallet/bdb.h>
 #include <wallet/db.h>