dfu: add prototype for dfu_transaction_initiate/cleanup

Add prototype for function dfu_transaction_initiate and
dfu_transaction_cleanup to avoid warning with W=1.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/include/dfu.h b/include/dfu.h
index fb5260d..2f0e335 100644
--- a/include/dfu.h
+++ b/include/dfu.h
@@ -209,6 +209,9 @@
  */
 void dfu_flush_callback(struct dfu_entity *dfu);
 
+int dfu_transaction_initiate(struct dfu_entity *dfu, bool read);
+void dfu_transaction_cleanup(struct dfu_entity *dfu);
+
 /*
  * dfu_defer_flush - pointer to store dfu_entity for deferred flashing.
  *		     It should be NULL when not used.