Fix Android compilation

[kasperisager]

I was seeing errors like these prior to this patch:

env/io_posix.cc:174:17: error: variable has incomplete type 'struct statfs'
  struct statfs buf;

env/io_posix.h:40:9: error: 'POSIX_MADV_NORMAL' macro redefined [-Werror,-Wmacro-redefined]
#define POSIX_MADV_NORMAL 0     /* [MC1] no further special treatment */

[kasperisager]

I'm still seeing the following error for 32-bit builds:

env/io_posix.cc:1380:39: error: comparison of integers of different signs: 'long long' and 'unsigned long long' [-Werror,-Wsign-compare]
  file_stats.st_blksize !=

[kasperisager]

This is still an issue as of v9.7.4: https://github.com/holepunchto/librocksdb/actions/runs/11954328841/job/33324183835

[facebook-github-bot]

@cbi42 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.