hs20: set PKG_SOURCE_SUBDIR

Building without the mirror-tarballs fails to PKG_SOURCE_SUBDIR not
matching the hostapd source subdir name. Fix that.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

net/hs20/Makefile

@@ -12,6 +12,7 @@ PKG_SOURCE_NAME:=hostapd
 PKG_MIRROR_HASH:=1b2a4947034142587a3f81eac2ea27dd64650e77f8943d973623b47533be2393
 PKG_MIRROR_VERSION:=$(PKG_SOURCE_DATE)-$(call version_abbrev,$(PKG_SOURCE_VERSION))
 PKG_SOURCE:=$(PKG_SOURCE_NAME)-$(PKG_MIRROR_VERSION).tar.xz
+PKG_SOURCE_SUBDIR:=$(PKG_SOURCE_NAME)-$(PKG_MIRROR_VERSION)
 PKG_BUILD_DIR:=$(BUILD_DIR)/$(PKG_NAME)/$(PKG_SOURCE_NAME)-$(PKG_MIRROR_VERSION)
 
 PKG_MAINTAINER:=Daniel Golle <daniel@makrotopia.org>