Merge pull request #10998 from jan-bruckner/mbedtls-3.6

Backport 3.6: Add linkage guard to constant_time.h

ChangeLog.d/add-linkage-guard.txt

@@ -0,0 +1,4 @@
+Bugfix
+   * Add extern "C" guard to constant_time.h. Fixes
+     https://github.com/Mbed-TLS/TF-PSA-Crypto/issues/930.
+

include/mbedtls/constant_time.h

@@ -11,6 +11,10 @@
 
 #include <stddef.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** Constant-time buffer comparison without branches.
  *
  * This is equivalent to the standard memcmp function, but is likely to be
@@ -33,4 +37,8 @@ int mbedtls_ct_memcmp(const void *a,
                       const void *b,
                       size_t n);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* MBEDTLS_CONSTANT_TIME_H */