signal.h: add linux/signal.h and asm/signal.h to UAPI compile-test coverage

linux/signal.h and asm/signal.h are currently excluded from the UAPI
compile-test because of the errors like follows:

    HDRTEST usr/include/asm/signal.h
  In file included from <command-line>:
  ./usr/include/asm/signal.h:103:9: error: unknown type name ‘size_t’
    103 |         size_t ss_size;
        |         ^~~~~~

The errors can be fixed by replacing size_t with __kernel_size_t.

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

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>