extras/fixture: add missing C++ include guards

This fixes linking errors when test cases based on Unity fixture are
defined in a .cpp file.

unity_internals.h doesn't have C++ guards, and is included from
unity.h from within C++ header guard block. Same approach is taken in
this commit

extras/fixture/src/unity_fixture.h

@@ -9,13 +9,20 @@
 #define UNITY_FIXTURE_H_
 
 #include "unity.h"
-#include "unity_internals.h"
 #include "unity_fixture_internals.h"
 
 #ifndef UNITY_FIXTURE_NO_EXTRAS
 #include "unity_memory.h"
 #endif
 
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+#include "unity_internals.h"
+
+
 int UnityMain(int argc, const char* argv[], void (*runAllTests)(void));
 
 
@@ -80,4 +87,8 @@ int UnityMain(int argc, const char* argv[], void (*runAllTests)(void));
 #define DOUBLES_EQUAL(expected, actual, delta)         TEST_ASSERT_DOUBLE_WITHIN((delta), (expected), (actual))
 #endif
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* UNITY_FIXTURE_H_ */