tests/benchmarks/footprints: Define required source standard macros

strnlen is not a C standard API, but an extension.
Instead of relaying that the SOURCE macro was set somewhere else
of that the C library provides a prototype anyhow
let's explicitly define it for this library.

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

tests/benchmarks/footprints/src/libc.c

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