Maybe fix mac build

Mac builds don't have posix_fallocate, so only define this for
Linux builds.

Fixes the following build error:

  external/sqlite/dist/sqlite3.c:27063:42: error: use of undeclared
  identifier 'posix_fallocate'
    { "fallocate",    (sqlite3_syscall_ptr)posix_fallocate,  0 },

Change-Id: Ie530861069d2efbc55bbe1e4e622094798cbe869