Update LIBUNWIND_ENABLE_WERROR default value in BuildingLibunwind.rst (#135546)

`LIBUNWIND_ENABLE_WERROR` defaults to `OFF` according to
[CMakeLists.txt](https://github.com/llvm/llvm-project/blob/main/libunwind/CMakeLists.txt#L43).

Author: firasuke

libunwind/docs/BuildingLibunwind.rst

@@ -91,7 +91,7 @@ libunwind specific options
 
 .. option:: LIBUNWIND_ENABLE_WERROR:BOOL
 
-  **Default**: ``ON``
+  **Default**: ``OFF``
 
   Compile with -Werror