Disambiguate version.h in doxygen comment

Specify mbedtls/version.h, since we are about to add
include/tf-psa-crypto/version.h.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/include/mbedtls/version.h b/include/mbedtls/version.h
index 637f9d3..718e99e 100644
--- a/include/mbedtls/version.h
+++ b/include/mbedtls/version.h
@@ -1,5 +1,5 @@
 /**
- * \file version.h
+ * \file mbedtls/version.h
  *
  * \brief Run-time version information
  */