Fix build on ARM, PPC

Those architectures use unsigned char by default, and command variable is later compared to -1.

src/tests/p11test/p11test.c

@@ -49,7 +49,7 @@ void display_usage() {
 }
 
 int main(int argc, char** argv) {
-	char command;
+	signed char command;
 	const struct CMUnitTest readonly_tests_without_initialization[] = {
 		/* Test card events on slot */
 		cmocka_unit_test_setup_teardown(wait_test,