Tests: Fix -Wstrict-prototypes warnings in some C sources

Some RunCMake tests fail with this warning due to extra stderr content:

    warning: this old-style function definition is not preceded by a prototype

Convert `foo()` to `foo(void)` in `.c` sources of affected tests.

Tests/RunCMake/ObjectLibrary/b.c

@@ -5,7 +5,7 @@
 #endif
 
 extern int a(void);
-EXPORT int b()
+EXPORT int b(void)
 {
   return a();
 }

Tests/RunCMake/ObjectLibrary/requires.c

@@ -1,5 +1,5 @@
 #ifdef REQUIRED
-int required()
+int required(void)
 {
   return 0;
 }

Tests/RunCMake/PrecompileHeaders/empty.c

@@ -1,3 +1,3 @@
-void nothing()
+void nothing(void)
 {
 }