pstore: Remove unused vmalloc.h in pmsg

Since the vmalloc code has been removed from write_pmsg() in the commit
"5bf6d1b9

 pstore/pmsg: drop bounce buffer", remove the unused header
vmalloc.h.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>