perf config: Allow '_' in config file variable names

For annotate I want to be able to have variables that are the same as
the ones representing feature toggles.

Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/perf/util/config.c

@@ -120,7 +120,7 @@ static char *parse_value(void)
 
 static inline int iskeychar(int c)
 {
-	return isalnum(c) || c == '-';
+	return isalnum(c) || c == '-' || c == '_';
 }
 
 static int get_value(config_fn_t fn, void *data, char *name, unsigned int len)