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/ecdsa.h b/include/mbedtls/ecdsa.h
index 4057828..6d99b97 100644
--- a/include/mbedtls/ecdsa.h
+++ b/include/mbedtls/ecdsa.h
@@ -32,6 +32,12 @@
 #ifndef MBEDTLS_ECDSA_H
 #define MBEDTLS_ECDSA_H
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
 #include "ecp.h"
 #include "md.h"