tests: Fix build with -Wextra

Unused arguments warnings are treated as errors in those tests
otherwise.

Fixes: #53.

Signed-off-by: Xavier Claessens <xavier.claessens@collabora.com>

tests/build-cxx.cc.in

@@ -8,5 +8,7 @@ using namespace std;
 int
 main(int argc, char **argv)
 {
+	(void)argc;
+	(void)argv;
 	return 0;
 }

tests/build-pedantic.c.in

@@ -6,5 +6,7 @@
 int
 main(int argc, char **argv)
 {
+	(void)argc;
+	(void)argv;
 	return 0;
 }