Added placeholders for MADV defines (#10881)

Summary:
Cross compiling rocksdb with rust bindings to android leads to an error since 7.4.0 (Incusion of madvise)
This is due to missing placeholders for non-linux platforms.

This PR adds the missing placeholders.

See rust-rocksdb/rust-rocksdb#697 for the specific error thrown.

I have just completed the CLA :)

Pull Request resolved: #10881

Reviewed By: akankshamahajan15

Differential Revision: D40726103

Pulled By: ajkr

fbshipit-source-id: 6b391636a74ef7e20d0daf47d332ddf0c14d5c34

env/io_posix.h

@@ -35,6 +35,12 @@
 #define POSIX_FADV_SEQUENTIAL 2 /* [MC1] expect sequential page refs */
 #define POSIX_FADV_WILLNEED 3   /* [MC1] will need these pages */
 #define POSIX_FADV_DONTNEED 4   /* [MC1] don't need these pages */
+
+#define POSIX_MADV_NORMAL 0     /* [MC1] no further special treatment */
+#define POSIX_MADV_RANDOM 1     /* [MC1] expect random page refs */
+#define POSIX_MADV_SEQUENTIAL 2 /* [MC1] expect sequential page refs */
+#define POSIX_MADV_WILLNEED 3   /* [MC1] will need these pages */
+#define POSIX_MADV_DONTNEED 4   /* [MC1] don't need these pages */
 #endif
 
 namespace ROCKSDB_NAMESPACE {