tools: perf: Fix build error in v4.19.y

perf may fail to build in v4.19.y with the following error.

util/evsel.c: In function ‘perf_evsel__exit’:
util/util.h:25:28: error:
	passing argument 1 of ‘free’ discards ‘const’ qualifier from pointer target type

This is observed (at least) with gcc v6.5.0. The underlying problem is
the following statement.
	zfree(&evsel->pmu_name);
evsel->pmu_name is decared 'const *'. zfree in turn is defined as
	#define zfree(ptr) ({ free(*ptr); *ptr = NULL; })
and thus passes the const * to free(). The problem is not seen
in the upstream kernel since zfree() has been rewritten there.

The problem has been introduced into v4.19.y with the backport of upstream
commit d4953f7ef1a2 (perf parse-events: Fix 3 use after frees found with
clang ASAN).

One possible fix of this problem would be to not declare pmu_name
as const. This patch chooses to typecast the parameter of zfree()
to void *, following the guidance from the upstream kernel which
does the same since commit 7f7c536f23e6a ("tools lib: Adopt
zalloc()/zfree() from tools/perf")

Fixes: a0100a3630 ("perf parse-events: Fix 3 use after frees found with clang ASAN")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/perf/util/util.h

@@ -22,7 +22,7 @@ static inline void *zalloc(size_t size)
 	return calloc(1, size);
 }
 
-#define zfree(ptr) ({ free(*ptr); *ptr = NULL; })
+#define zfree(ptr) ({ free((void *)*ptr); *ptr = NULL; })
 
 struct dirent;
 struct nsinfo;