drivers/entropy/native: Define required SOURCE macro

The srandom function is not available
unless _XOPEN_SOURCE is set > 500 or an equivalent
declaration is done.
Let's set it.

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

drivers/entropy/fake_entropy_native_bottom.c

@@ -7,6 +7,9 @@
  * ARCH_POSIX architecture
  */
 
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+
 #include <stdbool.h>
 #include <stdlib.h>
 #include <errno.h>