Include <span> where the split moved its only use

The #2546 test case guards itself with __has_include(<span>), but the
include itself sat in unit-regression2.cpp's preamble and stayed behind,
so the section compiled without a declaration wherever the guard passed -
which nvhpc reported and libc++ builds do not, as they skip the section
altogether.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-regression3.cpp

@@ -27,6 +27,7 @@ using json = nlohmann::json;
 using ordered_json = nlohmann::ordered_json;
 
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
 #include <limits>
 #include <string>
@@ -60,6 +61,12 @@ using ordered_json = nlohmann::ordered_json;
     >;
 #endif
 
+#ifdef JSON_HAS_CPP_20
+    #if __has_include(<span>)
+        #include <span>
+    #endif
+#endif
+
 #if JSON_HAS_RANGES == 1
     #include <ranges>
 #endif