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/include/mbedtls/version.h b/include/mbedtls/version.h
index 79b42b2..de67db3 100644
--- a/include/mbedtls/version.h
+++ b/include/mbedtls/version.h
@@ -39,7 +39,7 @@
  * Major, Minor, Patchlevel
  */
 #define MBEDTLS_VERSION_MAJOR  2
-#define MBEDTLS_VERSION_MINOR  17
+#define MBEDTLS_VERSION_MINOR  18
 #define MBEDTLS_VERSION_PATCH  0
 
 /**
@@ -47,9 +47,9 @@
  *    MMNNPP00
  *    Major version | Minor version | Patch version
  */
-#define MBEDTLS_VERSION_NUMBER         0x02110000
-#define MBEDTLS_VERSION_STRING         "2.17.0"
-#define MBEDTLS_VERSION_STRING_FULL    "mbed TLS 2.17.0"
+#define MBEDTLS_VERSION_NUMBER         0x02120000
+#define MBEDTLS_VERSION_STRING         "2.18.0"
+#define MBEDTLS_VERSION_STRING_FULL    "mbed TLS 2.18.0"
 
 #if defined(MBEDTLS_VERSION_C)