Declare nullptr_t ourselves. (#80)

See GCC bug #114869 [0] -- the nullptr_t type is not actually specified
as being built-in. This is the way it's now declared in gcc's stddef.h.

[0]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114869

Signed-off-by: Amy Ringo <me@remexre.com>

Author: remexre

src/kernel/include/types.h

@@ -84,6 +84,12 @@ static_assert(sizeof(uaddr) == __SIZEOF_SIZE_T__, "incorrect size for uaddr");
 static_assert(alignof(uaddr) == __SIZEOF_SIZE_T__,
               "incorrect alignment for uaddr");
 
+/**
+ * The type of null pointers.
+ */
+
+typedef typeof(nullptr) nullptr_t;
+
 /**
  * Strict provenance functions. See [0] for more details, but these are
  * basically APIs for doing "funny things" to pointers that compile better on