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>

cmd/Kconfig

@@ -810,6 +810,7 @@ config CMD_UNLZ4
 config CMD_UNZIP
 	bool "unzip"
 	default y if CMD_BOOTI
+	select GZIP
 	help
 	  Uncompress a zip-compressed memory region.