wallet: sqlite: force sqlite3.h to be included by the cpp files

This way sqlite usage is explicit.

src/wallet/sqlite.h

@@ -8,10 +8,11 @@
 #include <sync.h>
 #include <wallet/db.h>
 
-#include <sqlite3.h>
-
 struct bilingual_str;
 
+struct sqlite3_stmt;
+struct sqlite3;
+
 namespace wallet {
 class SQLiteDatabase;