Include configuration file for all headers

Include the configuration file for all headers that were not
included.
diff --git a/include/mbedtls/pkcs12.h b/include/mbedtls/pkcs12.h
index 69f0417..d441357 100644
--- a/include/mbedtls/pkcs12.h
+++ b/include/mbedtls/pkcs12.h
@@ -24,6 +24,12 @@
 #ifndef MBEDTLS_PKCS12_H
 #define MBEDTLS_PKCS12_H
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
 #include "md.h"
 #include "cipher.h"
 #include "asn1.h"