dfu: Make DFU virtual backend SPL friendly

Define stub for dfu_*_virt function in SPL, because
CONFIG_SPL_DFU_VIRT is not defined.

This patch avoids compilation issue in dfu_fill_entity() when
CONFIG_SPL_DFU is activated because the dfu_fill_entity_virt()
function is not available.

Fixes: ec44cace4b8d2 ("dfu: add DFU virtual backend")
Reported-by: Marek Vasut <marex@denx.de>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Marek Vasut <marex@denx.de>
diff --git a/include/dfu.h b/include/dfu.h
index dcb9cd9..0792222 100644
--- a/include/dfu.h
+++ b/include/dfu.h
@@ -495,7 +495,7 @@
 }
 #endif
 
-#ifdef CONFIG_DFU_VIRT
+#if CONFIG_IS_ENABLED(DFU_VIRT)
 int dfu_fill_entity_virt(struct dfu_entity *dfu, char *devstr,
 			 char **argv, int argc);
 int dfu_write_medium_virt(struct dfu_entity *dfu, u64 offset,