gh-150907: Fix dynamic_annotations.h when built with C++ and Valgrind

Add extern "C++" scope for the C++ template.

Author: vstinner

Include/dynamic_annotations.h

@@ -461,6 +461,7 @@ int RunningOnValgrind(void);
 
 #if DYNAMIC_ANNOTATIONS_ENABLED != 0 && defined(__cplusplus)
 
+extern "C++" {
   /* _Py_ANNOTATE_UNPROTECTED_READ is the preferred way to annotate racey reads.
 
      Instead of doing
@@ -476,6 +477,8 @@ int RunningOnValgrind(void);
     _Py_ANNOTATE_IGNORE_READS_END();
     return res;
   }
+}
+
   /* Apply _Py_ANNOTATE_BENIGN_RACE_SIZED to a static variable. */
 #define _Py_ANNOTATE_BENIGN_RACE_STATIC(static_var, description)        \
     namespace {                                                       \

Misc/NEWS.d/next/C_API/2026-06-04-14-26-17.gh-issue-150907.CA91_B.rst

@@ -0,0 +1,2 @@
+Fix ``dynamic_annotations.h`` header file when built with C++ and Valgrind:
+add ``extern "C++" scope`` for the C++ template. Patch by Victor Stinner.