Make ar invocation more portable

armar doesn't understand the syntax without dash. OTOH, the syntax with dash
is the only one specified by POSIX, and it's accepted by GNU ar, BSD ar (as
bundled with OS X) and armar, so it looks like the most portable syntax.

fixes #386

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 9712b15..0f81efe 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,7 +5,8 @@
 Bugfix
    * Fix bug in mbedtls_mpi_add_mpi() that caused wrong results when the three
      arguments where the same (in-place doubling). Found and fixed by Janos
-     Follath.
+     Follath. #309
+   * Fix issue in Makefile that prevented building using armar. #386
 
 = mbed TLS 1.3.16 released 2016-01-05