oshmem: fix macro usage in pshmem.h

pshmem.h now includes shmem.h (since f46130c) and some macros were removed at that time.

Use the OSHMEM_HAVE_C11 macro (defined in shmem.h) instead of the
previous OSHMEMP_HAVE_C11 macrso previously defined in pshmem.h

Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>

Author: ggouaillardet