modules: fatfs: fix build warnings

Adds a missing include to avoid these warnings :
- zephyr/modules/fatfs/zfs_ffsystem.c:19:16: warning: implicit declaration
  of function 'k_malloc'; did you mean 'ff_memalloc'?
  [-Wimplicit-function-declaration]
- zephyr/modules/fatfs/zfs_ffsystem.c:19:16: warning: returning 'int' from
  a function with return type 'void *' makes pointer from integer without a
  cast [-Wint-conversion]
- zephyr/modules/fatfs/zfs_ffsystem.c:25:9: warning: implicit declaration
  of function 'k_free' [-Wimplicit-function-declaration]

Signed-off-by: Johan Lafon <johan.lafon@syslinbit.com>