Update library version to 2.18.0

Increase the SO versions of libmbedx509 and libmbedtls due to the
addition of fields in publicly visible (non-opaque) structs:
  - mbedtls_ssl_config
  - mbedtls_ssl_context
  - mbedtls_x509_crt
diff --git a/library/Makefile b/library/Makefile
index 60f3ae0..89c4128 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -35,8 +35,8 @@
 endif
 endif
 
-SOEXT_TLS=so.12
-SOEXT_X509=so.0
+SOEXT_TLS=so.13
+SOEXT_X509=so.1
 SOEXT_CRYPTO=so.3
 
 # Set AR_DASH= (empty string) to use an ar implementation that does not accept