modules: mbedtls: fix conditional compilation for MBEDTLS_PKCS1_V21

Fix the conditional compilation to use `CONFIG_MBEDTLS_PKCS1_V21`
instead of `CONFIG_MBEDTLS_PKCS1_V15` when enabling `MBEDTLS_PKCS1_V21`.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>

Author: ndrs-pst

modules/mbedtls/configs/config-mbedtls.h

@@ -384,7 +384,7 @@
 #define MBEDTLS_PKCS1_V15
 #endif
 
-#if defined(CONFIG_MBEDTLS_PKCS1_V15)
+#if defined(CONFIG_MBEDTLS_PKCS1_V21)
 #define MBEDTLS_PKCS1_V21
 #endif