agpgart.h: do not include <stdlib.h> from exported header

Commit 35d0f1d5 ("include/uapi/linux/agpgart.h: include stdlib.h in
userspace") included <stdlib.h> to fix the unknown size_t error, but
I do not think it is the right fix.

This header already uses __kernel_size_t a few lines below.

Replace the remaining size_t, and stop including <stdlib.h>.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>