libsymbols kallsyms: Move hex2u64 out of header

hex2u64 is a helper that's out of place in kallsyms.h as not being
kallsyms related. Move from kallsyms.h to the only user.

Committer notes:

Move it out of tools/lib/symbol/kallsyms.c as well, as we had to leave
it there in the previous patch lest we break the build.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lore.kernel.org/lkml/20200501221315.54715-4-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>