wallet: bdb: include bdb header from our implementation files only

This way the dependency can't sneak into other files without being noticed.

Forward-declare bdb classes as necessary.

src/wallet/bdb.cpp

@@ -18,6 +18,7 @@
 
 #include <stdint.h>
 
+#include <db_cxx.h>
 #include <sys/stat.h>
 
 // Windows may not define S_IRUSR or S_IWUSR. We define both

src/wallet/bdb.h

@@ -21,10 +21,13 @@
 #include <unordered_map>
 #include <vector>
 
-#include <db_cxx.h>
-
 struct bilingual_str;
 
+class DbEnv;
+class DbTxn;
+class Db;
+class Dbc;
+
 // This constant was introduced in BDB 4.0.14 and has never changed, but there
 // is a belt-and-suspenders check in the cpp file just in case.
 #define BDB_DB_FILE_ID_LEN 20 /* Unique file ID length. */

src/wallet/salvage.cpp

@@ -11,6 +11,8 @@
 #include <wallet/wallet.h>
 #include <wallet/walletdb.h>
 
+#include <db_cxx.h>
+
 namespace wallet {
 /* End of headers, beginning of key/value data */
 static const char *HEADER_END = "HEADER=END";