gcc: fix get_completion_scheduler for -Og

The get_completion_schedule failed to link with -Og.  There doesn't
appear to be a direct way to detect this with pre-processor flags.
Therefore, switch the detection so that only size-optimization results
in the template type (__OPTIMIZE_SIZE__ instead of __OPTIMIZE__).

Fixes: d5d5cab
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>

include/stdexec/execution.hpp

@@ -378,7 +378,7 @@ namespace stdexec {
   inline constexpr get_delegatee_scheduler_t get_delegatee_scheduler{};
   inline constexpr get_allocator_t get_allocator{};
   inline constexpr get_stop_token_t get_stop_token{};
-#if !STDEXEC_GCC() || defined(__OPTIMIZE__)
+#if !STDEXEC_GCC() || defined(__OPTIMIZE_SIZE__)
   template <__completion_tag _CPO>
   inline constexpr get_completion_scheduler_t<_CPO> get_completion_scheduler{};
 #else