add preprocessor to hide unused functions

Summary:
encodeVarintToIOBuf and decodeVarintFromCursor are not used when
FOLLY_HAVE_LIBLZMA and FOLLY_HAVE_LIBLZ4 are false, which will trigger
-Werror,-Wunused-function on compile on mobile.

Test Plan: unit test

Reviewed By: seanc@fb.com

Subscribers: meisner, trunkagent, seanc, folly-diffs@, yfeldblum

FB internal diff: D1864016

Signature: t1:1864016:1424702813:75535ea92ac16385ce5c272d93bba507ccbb67a1

folly/io/Compression.cpp

@@ -148,6 +148,8 @@ std::unique_ptr<IOBuf> NoCompressionCodec::doUncompress(
   return data->clone();
 }
 
+#if (FOLLY_HAVE_LIBLZ4 || FOLLY_HAVE_LIBLZMA)
+
 namespace {
 
 void encodeVarintToIOBuf(uint64_t val, folly::IOBuf* out) {
@@ -173,6 +175,8 @@ inline uint64_t decodeVarintFromCursor(folly::io::Cursor& cursor) {
 
 }  // namespace
 
+#endif  // FOLLY_HAVE_LIBLZ4 || FOLLY_HAVE_LIBLZMA
+
 #if FOLLY_HAVE_LIBLZ4
 
 /**