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/sha512.c b/library/sha512.c
index 986037a..119f07f 100644
--- a/library/sha512.c
+++ b/library/sha512.c
@@ -527,8 +527,8 @@
 static const unsigned char sha512_test_buf[3][113] =
 {
     { "abc" },
-    { "abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmn"
-      "hijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu" },
+    { ("abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmn"
+      "hijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu") },
     { "" }
 };