Fix Linux GCC build issue.

Author: PeterSolMS

src/coreclr/gc/gcpriv.h

@@ -4528,7 +4528,7 @@ class gc_heap
     PER_HEAP
     size_t gen2_removed_no_undo;
 
-#define INVALID_SAVED_PINNED_PLUG_INDEX (~0)
+#define INVALID_SAVED_PINNED_PLUG_INDEX ((size_t)~0)
 
     PER_HEAP
     size_t saved_pinned_plug_index;