s390: squash facilities_src.h into gen_facilities.c

s390: squash facilities_src.h into gen_facilities.c

We generally expect headers in arch/$(ARCH)/include/asm directory
are included from kernel sources, but facilities_src.h is not;
it is included from the arch/s390/tools/gen_facilities.c tool.

There is no reason to expose this header to the public include path.
Furthermore, facilities_src.h makes sure to be included only from
gen_facilities.c by the following:

  #ifndef S390_GEN_FACILITIES_C
  #error "This file can only be included by gen_facilities.c"
  #endif

This check can be removed by merging the two files.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>