kexec.h: add linux/kexec.h to UAPI compile-test coverage

linux/kexec.h is currently excluded from the UAPI compile-test because
of the errors like follows:

    HDRTEST usr/include/linux/kexec.h
  In file included from <command-line>:
  ./usr/include/linux/kexec.h:56:9: error: unknown type name ‘size_t’
     56 |         size_t bufsz;
        |         ^~~~~~
  ./usr/include/linux/kexec.h:58:9: error: unknown type name ‘size_t’
     58 |         size_t memsz;
        |         ^~~~~~

The errors can be fixed by replacing size_t with __kernel_size_t.

Then, remove the no-header-test entry from user/include/Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>