tools headers: Update linux/ctype.h with the kernel sources

To pick up the changes in:

  caabdd0f ("ctype.h: remove duplicate isdigit() helper")

Addressing this perf build warning:

  Warning: Kernel ABI header at 'tools/include/linux/ctype.h' differs from latest version at 'include/linux/ctype.h'
  diff -u tools/include/linux/ctype.h include/linux/ctype.h

And we need to continue using the combination of:

  inline __isdigit()
  #define isdigit() __isdigit

When the __has_builtin() thing isn't available, as it is a builtin in
older systems with it as a builtin but with compilers not hacinv
__has_builtin(), rendering the __has_builtin() check useless otherwise.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>