Update Mbed Crypto SO version

The recent update changed the Mbed Crypto SO version, get Mbed TLS in
sync.
diff --git a/library/Makefile b/library/Makefile
index bc9363d..6fc110c 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -37,7 +37,7 @@
 
 SOEXT_TLS=so.13
 SOEXT_X509=so.1
-SOEXT_CRYPTO=so.3
+SOEXT_CRYPTO=so.4
 
 # Set AR_DASH= (empty string) to use an ar implementation that does not accept
 # the - prefix for command line options (e.g. llvm-ar)