xtest: fix compilation issue

This fixes the following compilation error:

external/optee_test/host/xtest/sdp_basic.c:687:91: error: unused parameter 'ion_heap' [-Werror,-Wunused-parameter]
static void usage(const char *progname, size_t size, int loop, const char *heap_name, int ion_heap)

Signed-off-by: Pierre Moos <pmoos@baylibre.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/host/xtest/sdp_basic.c b/host/xtest/sdp_basic.c
index 2b09a5b..16241c4 100644
--- a/host/xtest/sdp_basic.c
+++ b/host/xtest/sdp_basic.c
@@ -700,6 +700,8 @@
 	fprintf(stderr, " -n LOOP           Test loop iterations [%u]\n", loop);
 #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 11, 0)
 	fprintf(stderr, " --heap ID         Target heap ID [%d]\n", ion_heap);
+#else
+	(void)ion_heap;
 #endif
 	fprintf(stderr, " --heap-name NAME  Target heap name [%s]\n", heap_name);
 	fprintf(stderr, " --no-offset       No random offset [0 255] in buffer\n");