eal: fix build with musl

eal: fix build with musl

In musl libc, cpu_set_t is defined only if _GNU_SOURCE is defined.
In case _GNU_SOURCE is undefined, as in eal_common_errno.c,
it was not possible to include rte_os.h which uses cpu_set_t.

This limitation is removed: if CPU_SETSIZE is not defined,
cpu_set_t related definitions and functions are skipped.
Note: such definitions are unneeded in eal_common_errno.c.

Applications which do not define _GNU_SOURCE may miss cpu_set_t related
features on musl. Such case is detected by RTE_HAS_CPUSET being undefined,
so functions which depend on rte_cpuset_t will be unavailable.

A missing include of fcntl.h is also added.

Bugzilla ID: 35
Fixes: 11b57c698005 ("eal: fix error string function")
Fixes: 176bb37ca6f3 ("eal: introduce internal wrappers for file operations")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: David Marchand <david.marchand@redhat.com>