image-fit: Make string of algo parameter constant

Modifications would be invalid.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/image-host.c b/tools/image-host.c
index 945571f..5b814ce 100644
--- a/tools/image-host.c
+++ b/tools/image-host.c
@@ -63,7 +63,7 @@
 	uint8_t value[FIT_MAX_HASH_LEN];
 	const char *node_name;
 	int value_len;
-	char *algo;
+	const char *algo;
 	int ret;
 
 	node_name = fit_get_name(fit, noffset, NULL);
@@ -160,7 +160,7 @@
 		const char *engine_id)
 {
 	const char *node_name;
-	char *algo_name;
+	const char *algo_name;
 	const char *padding_name;
 
 	node_name = fit_get_name(fit, noffset, NULL);