add PortableSpinLock.h to Makefile, unbreak fbthrift build

Summary:
fix fbthrift build, broken in D1621717

I wonder if it'd be possible to lint against this somehow. The jenkins failures are easy to miss especially on folly diffs with lots of failure noise (in this case it was still postponed)

Test Plan: wait for jenkins

Reviewed By: davejwatson@fb.com

Subscribers: njormrod, folly-diffs@

FB internal diff: D1630833

folly/Makefile.am

@@ -124,6 +124,7 @@ nobase_follyinclude_HEADERS = \
 	io/Cursor.h \
 	io/IOBuf.h \
 	io/IOBufQueue.h \
+	io/PortableSpinLock.h \
 	io/RecordIO.h \
 	io/RecordIO-inl.h \
 	io/TypedIOBuf.h \