Fix Android compilation (#12914)

Summary:
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 */
```

Pull Request resolved: #12914

Reviewed By: hx235

Differential Revision: D66307960

Pulled By: cbi42

fbshipit-source-id: f38034c56986e7877c9a04046b910bbeec9be1fe

env/io_posix.cc

@@ -28,7 +28,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
-#ifdef OS_LINUX
+#if defined(OS_LINUX) || defined(OS_ANDROID)
 #include <sys/statfs.h>
 #include <sys/sysmacros.h>
 #endif
@@ -1421,9 +1421,10 @@ IOStatus PosixWritableFile::Close(const IOOptions& /*opts*/,
     // After ftruncate, we check whether ftruncate has the correct behavior.
     // If not, we should hack it with FALLOC_FL_PUNCH_HOLE
     if (result == 0 &&
-        (file_stats.st_size + file_stats.st_blksize - 1) /
-                file_stats.st_blksize !=
-            file_stats.st_blocks / (file_stats.st_blksize / 512)) {
+        static_cast<size_t>((file_stats.st_size + file_stats.st_blksize - 1) /
+                            file_stats.st_blksize) !=
+            static_cast<size_t>(file_stats.st_blocks /
+                                (file_stats.st_blksize / 512))) {
       IOSTATS_TIMER_GUARD(allocate_nanos);
       if (allow_fallocate_) {
         fallocate(fd_, FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE, filesize_,

env/io_posix.h

@@ -30,7 +30,7 @@
 // For non linux platform, the following macros are used only as place
 // holder.
 #if !(defined OS_LINUX) && !(defined OS_FREEBSD) && !(defined CYGWIN) && \
-    !(defined OS_AIX)
+    !(defined OS_AIX) && !(defined OS_ANDROID)
 #define POSIX_FADV_NORMAL 0     /* [MC1] no further special treatment */
 #define POSIX_FADV_RANDOM 1     /* [MC1] expect random page refs */
 #define POSIX_FADV_SEQUENTIAL 2 /* [MC1] expect sequential page refs */