Hide getGCCycles when we have no GC

Alternatively, we could make it return 0, but we don't need it in
the first place because the caller exists conditionally too.

src/libexpr/eval-gc.hh

@@ -15,9 +15,11 @@ void initGC();
  */
 void assertGCInitialized();
 
+#ifdef HAVE_BOEHMGC
 /**
  * The number of GC cycles since initGC().
  */
 size_t getGCCycles();
+#endif
 
-}
+} // namespace nix