Fixes compile error that highlights problem:

/source/storage/innobase/fil/fil0crypt.cc: In function 'void fil_crypt_rotate_page(const key_state_t*, rotate_thread_t*)':
/source/storage/innobase/fil/fil0crypt.cc:1770:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
  if (space == TRX_SYS_SPACE && offset == TRX_SYS_PAGE_NO) {
Signed-off-by: Daniel Black <daniel.black@au.ibm.com>