spsc_q: fix use of stdatomic.h

stdatomic.h is a C++23 feature [1]. Thus, "older" compilers such as
gcc-11 (default in Ubuntu 22) do not know this, and can therefore not
correctly compile spsc_q. Fix this by alternatively including atomic for
C++.

Fixes errors such as

    stdatomic.h:40:9: error: ‘_Atomic’ does not name a type

[1] https://en.cppreference.com/cpp/header/stdatomic.h

Signed-off-by: Robert Schmidt <robert.schmidt@openairinterface.org>

common/utils/ds/spsc_q.h

@@ -7,11 +7,13 @@
 
 #include <stdint.h>
 #include <stdbool.h>
-#include <stdatomic.h>
 
 #if defined(__cplusplus)
+#include <atomic>
 // use atomic_size_t to help interworking with C++
 using std::atomic_size_t;
+#else
+#include <stdatomic.h>
 #endif
 
 typedef struct spsc_q {