Prevent triggering Clang 12 -Wstring-concatenation warning

Wrap multi-line string literals in parentheses
to prevent a Clang 12 -Wstring-concatenation warning
(activated by -Wall), which caused the build to fail.

Fixes https://github.com/ARMmbed/mbedtls/issues/3586

Signed-off-by: Guido Vranken <guidovranken@gmail.com>
diff --git a/library/ripemd160.c b/library/ripemd160.c
index d6ee933..77acfc7 100644
--- a/library/ripemd160.c
+++ b/library/ripemd160.c
@@ -514,8 +514,8 @@
     { "abcdefghijklmnopqrstuvwxyz" },
     { "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq" },
     { "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789" },
-    { "12345678901234567890123456789012345678901234567890123456789012"
-      "345678901234567890" },
+    { ("12345678901234567890123456789012345678901234567890123456789012"
+      "345678901234567890") },
 };
 
 static const size_t ripemd160_test_strlen[TESTS] =