Add missing escape character in multi-line preprocessor directive

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 402da8d..13c331f 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -162,7 +162,7 @@
 #define MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC
 #endif
 
-#if defined(MBEDTLS_SSL_SOME_SUITES_USE_STREAM) ||
+#if defined(MBEDTLS_SSL_SOME_SUITES_USE_STREAM) || \
     defined(MBEDTLS_SSL_SOME_SUITES_USE_CBC)
 #define MBEDTLS_SSL_SOME_SUITES_USE_MAC
 #endif