Fixing compiler warnings on ubuntu 12.10

When compiling on Ubuntu 12.10 (GCC version 4.7.2 (Ubuntu/Linaro 4.7.2-2ubuntu1)) , getting the following warnings:
```gcc
CC rdb.o
rdb.c: In function ‘rdbSaveBackground’:
rdb.c:742:21: warning: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘size_t’ [-Wformat]
```
```gcc
CC aof.o
aof.c: In function ‘rewriteAppendOnlyFileBackground’:
aof.c:972:21: warning: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘size_t’ [-Wformat]
```

In order to fix it -- I have done a cast in the mentioned places (it has seemed to me the correct way to fix it).