Fix gcc warning in posix port

Fix warning from "gcc -Wsign-compare" in the file
portable/ThirdParty/GCC/Posix/port.c since PTHREAD_STACK_MIN
is used from system headers.

Signed-off-by: Florian La Roche <Florian.LaRoche@gmail.com>

Author: laroche

portable/ThirdParty/GCC/Posix/port.c

@@ -180,7 +180,7 @@ StackType_t * pxPortInitialiseStack( StackType_t * pxTopOfStack,
     thread->xDying = pdFALSE;
 
     /* Ensure ulStackSize is at least PTHREAD_STACK_MIN */
-    ulStackSize = (ulStackSize < PTHREAD_STACK_MIN) ? PTHREAD_STACK_MIN : ulStackSize;
+    ulStackSize = (ulStackSize < ( size_t ) ( PTHREAD_STACK_MIN ) ) ? ( size_t ) ( PTHREAD_STACK_MIN ) : ulStackSize;
 
     pthread_attr_init( &xThreadAttributes );
     iRet = pthread_attr_setstacksize( &xThreadAttributes, ulStackSize );