AP_Common: add directory to add missing C/C++ symbols

Add a cstddef header to allow using std::nullptr_t with those toolchains
that don't provide it. The idea is to make these platforms to use our
wrapper header (see https://gcc.gnu.org/onlinedocs/cpp/Wrapper-Headers.html)
and then we add the missing bits to the header.

libraries/AP_Common/missing/cstddef

@@ -0,0 +1,9 @@
+#pragma once
+
+#include_next <cstddef>
+
+#if defined(HAVE_NULLPTR_T) && !HAVE_NULLPTR_T
+namespace std {
+    typedef decltype(nullptr) nullptr_t;
+}
+#endif