Fix: avoid set but unused variable warning in pxPortInitialiseStack (#1320)

diff --git a/portable/ThirdParty/GCC/Posix/port.c b/portable/ThirdParty/GCC/Posix/port.c
index 2342f60..a1b1ca8 100644
--- a/portable/ThirdParty/GCC/Posix/port.c
+++ b/portable/ThirdParty/GCC/Posix/port.c
@@ -229,6 +229,7 @@
     /* Ensure that there is enough space to store Thread_t on the stack. */
     ulStackSize = ( size_t ) ( pxTopOfStack + 1 - pxEndOfStack ) * sizeof( *pxTopOfStack );
     configASSERT( ulStackSize > sizeof( Thread_t ) );
+    ( void ) ulStackSize; /* suppress set but not used warning */
 
     thread->pxCode = pxCode;
     thread->pvParams = pvParameters;