macOS: add config file search path

this way the default config file is used in brew package if no config file is
set

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

config/config_linux.go

@@ -23,4 +23,5 @@ import "github.com/spf13/viper"
 func setViperAdditionalConfigPaths() {
 	viper.AddConfigPath("$HOME/.config/sftpgo")
 	viper.AddConfigPath("/etc/sftpgo")
+	viper.AddConfigPath("/usr/local/etc/sftpgo")
 }