equeue: Enable use of stub from C++

Add extern "C" to the equeue_stub declaration to avoid an error when a C
file implements the equeue_stub symbol (as we do in equeue_stub.c).

    Undefined symbols for architecture x86_64:
      "_equeue_stub", referenced from:
          Test_LoRaWANTimer_start_Test::TestBody() in Test_LoRaWANTimer.cpp.o
    ld: symbol(s) not found for architecture x86_64
    clang: error: linker command failed with exit code 1 (use -v to see invocation)

events/tests/UNITTESTS/doubles/equeue_stub.h

@@ -21,6 +21,10 @@
 #include "stdint.h"
 #include "stdbool.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef struct {
     void *void_ptr;
     bool call_cb_immediately;
@@ -28,4 +32,8 @@ typedef struct {
 
 extern equeue_stub_def equeue_stub;
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif