examples/posix_stdio: Fix Kconfig to define default stack size

Fix the Kconfig file for the posix_stdio example by adding
the STACKSIZE symbol and setting it to DEFAULT_TASK_STACKSIZE.

This change ensures that the example builds properly and
respects system-wide stack size configuration.

Signed-off-by: Vinicius May <vmay.sweden@gmail.com>
diff --git a/examples/posix_stdio/Kconfig b/examples/posix_stdio/Kconfig
index 3c41e97..8980223 100644
--- a/examples/posix_stdio/Kconfig
+++ b/examples/posix_stdio/Kconfig
@@ -7,7 +7,8 @@
 	bool "Posix stdio example"
 	default n
 	---help---
-    	Enable POSIX stdio example that shows how to use open(), write() and close() via /dev/console.
+		Enable POSIX stdio example that shows how to use open(), write() and
+		close() via /dev/console.
 
 config EXAMPLES_POSIX_STDIO_PROGNAME
 	string "Program name"
@@ -15,6 +16,11 @@
 	depends on EXAMPLES_POSIX_STDIO
 
 config EXAMPLES_POSIX_STDIO_PRIORITY
-	int "POSIX_STDIO test priority"
+	int "POSIX_STDIO priority"
 	default 100
 	depends on EXAMPLES_POSIX_STDIO
+
+config EXAMPLES_POSIX_STDIO_STACKSIZE
+	int "POSIX_STDIO stack size"
+	default DEFAULT_TASK_STACKSIZE
+	depends on EXAMPLES_POSIX_STDIO