fix a compiler sign warning in OpenBlockFile()

src/main.cpp

@@ -1983,7 +1983,7 @@ FILE* AppendBlockFile(unsigned int& nFileRet)
         if (fseek(file, 0, SEEK_END) != 0)
             return NULL;
         // FAT32 file size max 4GB, fseek and ftell max 2GB, so we must stay under 2GB
-        if (ftell(file) < 0x7F000000 - MAX_SIZE)
+        if (ftell(file) < (long)(0x7F000000 - MAX_SIZE))
         {
             nFileRet = nCurrentBlockFile;
             return file;