[EFR32] Fix missing include of stdio when using snprintf in mbedtls (#37763)

Usage of MBEDTLS_PLATFORM_NO_STD_FUNCTIONS means the mbedtls headers are
not including stdio by default, leading to usage of snprintf without prior
declaration in x509_csr.c.

Author: stevew817

src/platform/silabs/efr32/efr32-chip-mbedtls-config.h

@@ -95,6 +95,8 @@
 #define MBEDTLS_ECDSA_DETERMINISTIC
 #endif // SL_MATTER_PROVISION_FLASH
 
+// If defining snprintf we are also responsible for including its declaration
+#include <stdio.h>
 #define MBEDTLS_PLATFORM_SNPRINTF_MACRO snprintf
 
 #define MBEDTLS_AES_ROM_TABLES