This prototype is obtained indirectly, by luck, from some other header
in probe-event.c in most systems, but recently exploded on alpine:edge:

   8    13.39 alpine:edge                   : FAIL gcc version 13.2.1 20240309 (Alpine 13.2.1_git20240309)
    util/probe-event.c: In function 'convert_exec_to_group':
    util/probe-event.c:225:16: error: implicit declaration of function 'basename' [-Werror=implicit-function-declaration]
      225 |         ptr1 = basename(exec_copy);
          |                ^~~~~~~~
    util/probe-event.c:225:14: error: assignment to 'char *' from 'int' makes pointer from integer without a cast [-Werror=int-conversion]
      225 |         ptr1 = basename(exec_copy);
          |              ^
    cc1: all warnings being treated as errors
    make[3]: *** [/git/perf-6.8.0/tools/build/Makefile.build:158: util] Error 2

Fix it by adding the libgen.h header where basename() is prototyped.

Fixes: fb7345bb ("perf probe: Support basic dwarf-based operations on uprobe events")
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>