Include configuration file to headers

Add inclusion to configration file in header files,
instead of relying on other header files to include
the configuration file. This issue resolves #1371
diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index f76fc80..dc6947b 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -24,6 +24,12 @@
 #ifndef MBEDTLS_ASN1_WRITE_H
 #define MBEDTLS_ASN1_WRITE_H
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
 #include "asn1.h"
 
 #define MBEDTLS_ASN1_CHK_ADD(g, f) do { if( ( ret = f ) < 0 ) return( ret ); else   \