Properly conditionalize DwarfFDECache<A>::dyldUnloadHook. Patch by Patrick Wildt

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@206121 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/Unwind/UnwindCursor.hpp b/src/Unwind/UnwindCursor.hpp
index 235922f..aa14dd1 100644
--- a/src/Unwind/UnwindCursor.hpp
+++ b/src/Unwind/UnwindCursor.hpp
@@ -151,10 +151,12 @@
   _LIBUNWIND_LOG_NON_ZERO(::pthread_rwlock_unlock(&_lock));
 }
 
+#if __APPLE__
 template <typename A>
 void DwarfFDECache<A>::dyldUnloadHook(const struct mach_header *mh, intptr_t ) {
   removeAllIn((pint_t) mh);
 }
+#endif
 
 template <typename A>
 void DwarfFDECache<A>::iterateCacheEntries(void (*func)(