Tests: Fix clang -Wstrict-prototypes warnings

Tests/Module/WriteCompilerDetectionHeader/c_undefined.c

@@ -1,7 +1,7 @@
 
 #include "test_compiler_detection.h"
 
-int main()
+int main(void)
 {
   return 0;
 }

Tests/Module/WriteCompilerDetectionHeader/main.c

@@ -24,7 +24,7 @@
 #  error Expect no CXX features defined
 #endif
 
-int main()
+int main(void)
 {
   return 0;
 }

Tests/Module/WriteCompilerDetectionHeader/main_multi.c

@@ -24,7 +24,7 @@
 #  error Expect no CXX features defined
 #endif
 
-int main()
+int main(void)
 {
   return 0;
 }