Add foundation header to ActivityStreamSPI. (#461)

* Add C++ ifdefs around C++ code.

The purpose of this change is to gate C++ code behind a cplusplus define so that FLEX can be made into a module for Swift code.  Swift modules do not allow for C++ code.  However this means if FLEX is used a Swift module, it will not have functionality of the excluded code. Ideally in the future, FLEX can be converted fully to support Swift without bridging headers.

* Revert "Add C++ ifdefs around C++ code."

This reverts commit 77c02207f9aa5f2325599ebd70032a380fbb1fe6.

* Add include <Foundation/Foundation.h> to ActivityStreamSPI header

We use a different build setup internally, and we need all headers to be able to stand on their own. This file is using NS_ENUM, which is defined in Foundation, without importing it, so we're adding the import to resolve that.

Classes/GlobalStateExplorers/SystemLog/ActivityStreamAPI.h

@@ -14,6 +14,8 @@
 #ifndef ActivityStreamSPI_h
 #define ActivityStreamSPI_h
 
+#include <Foundation/Foundation.h>
+
 #include <sys/time.h>
 // #include <xpc/xpc.h>