psa: Fix builds without MBEDTLS_PLATFORM_C

When `MBEDTLS_PLATFORM_C` is not enabled, our PSA Crypto implementation
depends on the standard C library for functions like snprintf() and
exit(). However, our implementation was not including the proper header
files nor redefining all `mbedtls_*` symbols properly to ensure
successful builds without MBEDTLS_PLATFORM_C. Add the necessary header
files and macro definitions to our PSA Crypto implementation.
diff --git a/programs/psa/key_ladder_demo.c b/programs/psa/key_ladder_demo.c
index 0943bf5..c9d7676 100644
--- a/programs/psa/key_ladder_demo.c
+++ b/programs/psa/key_ladder_demo.c
@@ -66,6 +66,7 @@
 #define mbedtls_calloc       calloc
 #define mbedtls_free         free
 #define mbedtls_printf       printf
+#define mbedtls_exit         exit
 #endif
 #include <stdio.h>
 #include <string.h>