allow building without sqlite3

Changelog-Changed: build: SQLite3 is no longer a hard build requirement. C-Lightning can now be built to support only the PostgreSQL back-end.

wallet/db_common.h

@@ -4,7 +4,6 @@
 #include <ccan/autodata/autodata.h>
 #include <ccan/list/list.h>
 #include <ccan/short_types/short_types.h>
-#include <sqlite3.h>
 #include <stdbool.h>
 
 /* For testing, we want to catch fatal messages. */

wallet/db_postgres.c

@@ -3,7 +3,6 @@
 #include <ccan/ccan/tal/str/str.h>
 #include <ccan/endian/endian.h>
 #include <lightningd/log.h>
-#include <sqlite3.h>
 #include <stdio.h>
 
 #if HAVE_POSTGRES

wallet/db_sqlite3.c

@@ -2,10 +2,10 @@
 #include "gen_db_sqlite3.c"
 #include <ccan/ccan/tal/str/str.h>
 #include <lightningd/log.h>
-#include <sqlite3.h>
 #include <stdio.h>
 
 #if HAVE_SQLITE3
+#include <sqlite3.h>
 
 #if !HAVE_SQLITE3_EXPANDED_SQL
 /* Prior to sqlite3 v3.14, we have to use tracing to dump statements */