Tests: Add missing return types to C function prototypes

These were diagnosed by a Clang 15 development version.

Tests/ExportImport/Import/A/imp_testExe1.c

@@ -19,7 +19,7 @@ extern int testStaticLibPlugin(void);
 #else
 #  define testLib4libcfg testLib4libopt
 #endif
-extern testLib4libcfg(void);
+extern int testLib4libcfg(void);
 
 int main()
 {

Tests/TargetName/executables/hello_world.c

@@ -1,5 +1,6 @@
 #include <stdio.h>
-main()
+int main(void)
 {
   printf("hello, world\n");
+  return 0;
 }