perf_counter tools: fix Documentation/perf_counter build error

Mike Galbraith reported:

> marge:..Documentation/perf_counter # make
>     CC builtin-stat.o
> In file included from builtin-stat.c:71:
> /usr/include/ctype.h:102: error: expected expression before ‘]’ token

Remove the ctype.h include.

Signed-off-by: Ingo Molnar <mingo@elte.hu>