fsstorage: missing #include

```--
  CC       addons/fsstorage.lo
addons/fsstorage.c: In function 'WriteCatalog':
addons/fsstorage.c:543:5: error: implicit declaration of function 'getpid' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
make.exe[5]: *** [addons/fsstorage.lo] Error 1
```

--
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/misc/addons/fsstorage.c

@@ -40,6 +40,8 @@
 #include <sys/stat.h>
 #include <errno.h>
 
+#include <unistd.h>     // getpid()
+
 /*****************************************************************************
  * Local prototypes
  *****************************************************************************/