Add missing cstddef include for C++ builds

Closes GH-15096.

Author: cmb69

NEWS

@@ -6,6 +6,7 @@ PHP                                                                        NEWS
   . Fix GH-14978 (The xmlreader extension phpize build). (Peter Kokot)
   . Throw Error exception when encountering recursion during comparison, rather
     than fatal error. (ilutov)
+  . Added missing cstddef include for C++ builds. (cmb)
 
 - BCMath:
   . Adjust bcround()'s $mode parameter to only accept the RoundingMode

Zend/zend_portability.h

@@ -791,9 +791,13 @@ extern "C++" {
 # define ZEND_STATIC_ASSERT(c, m)
 #endif
 
-#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) /* C11 */ \
-  || (defined(__cplusplus) && __cplusplus >= 201103L) /* C++11 */
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) /* C11 */
 typedef max_align_t zend_max_align_t;
+#elif (defined(__cplusplus) && __cplusplus >= 201103L) /* C++11 */
+extern "C++" {
+# include <cstddef>
+}
+typedef std::max_align_t zend_max_align_t;
 #else
 typedef union {
 	char c;