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/md2.c b/library/md2.c
index fdcb630..13cb13f 100644
--- a/library/md2.c
+++ b/library/md2.c
@@ -320,8 +320,8 @@
     { "message digest" },
     { "abcdefghijklmnopqrstuvwxyz" },
     { "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789" },
-    { "12345678901234567890123456789012345678901234567890123456789012"
-      "345678901234567890" }
+    { ("12345678901234567890123456789012345678901234567890123456789012"
+      "345678901234567890") }
 };
 
 static const size_t md2_test_strlen[7] =