Add more parentheses for MSVC

Author: cschreib

include/oup/observable_unique_ptr.hpp

@@ -201,8 +201,8 @@ class basic_control_block final {
     static constexpr control_block_storage_type get_highest_bit_mask() {
         // NB: This is put in a function to avoid a spurious MSVC warning.
         return static_cast<control_block_storage_type>(1)
-               << static_cast<control_block_storage_type>(
-                      sizeof(control_block_storage_type) * 8 - 1);
+               << (static_cast<control_block_storage_type>(
+                      sizeof(control_block_storage_type) * 8 - 1));
     }
 
     static constexpr control_block_storage_type highest_bit_mask = get_highest_bit_mask();