Fix CM_FALLTHROUGH with -Wunused-parameter

Fix the test code that we `try_compile` to avoid unused parameter
warnings that cause the check to fail.

Source/Checks/cm_cxx_attribute_fallthrough.cxx

@@ -1,4 +1,4 @@
-int main(int argc, char* argv[])
+int main(int argc, char* [])
 {
   int i = 3;
   switch (argc) {

Source/Checks/cm_cxx_fallthrough.cxx

@@ -1,4 +1,4 @@
-int main(int argc, char* argv[])
+int main(int argc, char* [])
 {
   int i = 3;
   switch (argc) {

Source/Checks/cm_cxx_gnu_fallthrough.cxx

@@ -1,4 +1,4 @@
-int main(int argc, char* argv[])
+int main(int argc, char* [])
 {
   int i = 3;
   switch (argc) {