remsim-client: move from common 'src' directory to 'src/client'

As both the bankd and the server already are in src/bankd and
src/server, respectively: Let's unify this and have the client
also in its own sub-directory.

Change-Id: I67a3a5941434f09f7099c2cdb19c126cea305a73
diff --git a/src/bankd/bankd.h b/src/bankd/bankd.h
index d713ed1..6de3213 100644
--- a/src/bankd/bankd.h
+++ b/src/bankd/bankd.h
@@ -14,7 +14,7 @@
 
 #include "rspro_util.h"
 #include "slotmap.h"
-#include "client.h"
+#include "rspro_client_fsm.h"
 #include "debug.h"
 
 extern struct value_string worker_state_names[];