config: add `const` qualifier to set_pool_{algorithm,nfactor}().

MSVS doesn't like implicit casting.

sgminer.c

@@ -781,7 +781,7 @@ static char *set_url(char *arg)
 }
 
 
-static char *set_pool_algorithm(char *arg)
+static char *set_pool_algorithm(const char *arg)
 {
 	struct pool *pool = get_current_pool();
 
@@ -791,7 +791,7 @@ static char *set_pool_algorithm(char *arg)
 	return NULL;
 }
 
-static char *set_pool_nfactor(char *arg)
+static char *set_pool_nfactor(const char *arg)
 {
 	struct pool *pool = get_current_pool();