#3420 Declare MarkAsIgnored as a DLL export

This was causing the following linker error on Microsoft Visual C++ when compiling as a DLL:

```
googletest-param-test-test.cc.obj : error LNK2019: unresolved external symbol "public: __cdecl testing::internal::MarkAsIgnored::MarkAsIgnored(char const *)" (??0MarkAsIgnored@internal@testing@@QEAA@PEBD@Z) referenced in function "void __cdecl works_here::`dynamic initializer for 'gtest_allow_ignore_NotInstantiatedTest''(void)" (??__Egtest_allow_ignore_NotInstantiatedTest@works_here@@YAXXZ)
```

googletest/include/gtest/internal/gtest-param-util.h

@@ -478,7 +478,7 @@ class ParameterizedTestSuiteInfoBase {
 //
 // Report a the name of a test_suit as safe to ignore
 // as the side effect of construction of this type.
-struct MarkAsIgnored {
+struct GTEST_API_ MarkAsIgnored {
   explicit MarkAsIgnored(const char* test_suite);
 };