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 <[e-mail]>
Reviewed-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

include/uapi/linux/kexec.h

···
  */
 struct kexec_segment {
 	const void *buf;
-	size_t bufsz;
+	__kernel_size_t bufsz;
 	const void *mem;
-	size_t memsz;
+	__kernel_size_t memsz;
 };
 
 #endif /* __KERNEL__ */

usr/include/Makefile

···
 no-header-test += linux/errqueue.h
 no-header-test += linux/hdlc/ioctl.h
 no-header-test += linux/ivtv.h
-no-header-test += linux/kexec.h
 no-header-test += linux/matroxfb.h
 no-header-test += linux/omap3isp.h
 no-header-test += linux/omapfb.h