subsys/ipc icmsg_me.c: Fix ssize_t build error for some libCs

ssize_t is not part of the C library ISO standard subset, and as such
one cannot expect a C library to expose the type by default.
Some libraries like glibc do not expose this type in general, and
trying to build using them results in build errors.

There is 3 possible options:
1. Continue using ssize_t and:
1.a define _POSIX_C_SOURCE before including any header
1.b include sys/types.h directly (A bit uglier)
3. Not use ssize_t, and instead rely on ISO standard types

Let's just go with 1.a assuming we want to keep using this type,
as that is the correct way of getting this type defined.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

subsys/ipc/ipc_service/lib/icmsg_me.c

@@ -4,6 +4,9 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#undef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+
 #include <zephyr/ipc/icmsg_me.h>
 #include <zephyr/sys/math_extras.h>