perf symbols: Constify dso->long_name

perf symbols: Constify dso->long_name

Same reason as for dso->short_name, it may point to a const string, and
in most places it is treated as const, i.e. it is just accessed for
using its contents as a key or to show it on reports.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-nf7mxf33zt5qw207pbxxryot@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>