selftests: arm64: Fix printf() format mismatch in vec-syscfg

The format for this error message calls for the plain text version of the
error but we weren't supply it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210929151925.9601-2-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

tools/testing/selftests/arm64/fp/vec-syscfg.c

@@ -109,7 +109,7 @@ static int get_child_rdvl(struct vec_data *data)
 
 		/* exec() a new binary which puts the VL on stdout */
 		ret = execl(data->rdvl_binary, data->rdvl_binary, NULL);
-		fprintf(stderr, "execl(%s) failed: %d\n",
+		fprintf(stderr, "execl(%s) failed: %d (%s)\n",
 			data->rdvl_binary, errno, strerror(errno));
 
 		exit(EXIT_FAILURE);