cmd: unzip: automatically select CONFIG_GZIP

unzip calls gzwrite() which is provided in lib/gunzip.c. Make sure it is
automatically pulled in if the user selects CMD_UNZIP.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Heinrich Schuchardt <xypron.glkp@gmx.de>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index f9be198..f4eb575 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -810,6 +810,7 @@
 config CMD_UNZIP
 	bool "unzip"
 	default y if CMD_BOOTI
+	select GZIP
 	help
 	  Uncompress a zip-compressed memory region.