Fix AppVeyour VC warning:'>': signed/unsigned mismatch

contrib/extractor/mpq_libmpq.cpp

@@ -81,7 +81,7 @@ size_t MPQFile::read(void* dest, size_t bytes)
 {
     if (eof) return 0;
 
-    size_t rpos = pointer + bytes;
+    libmpq__off_t rpos = pointer + bytes;
     if (rpos > size)
     {
         bytes = size - pointer;