uboot-fritz4040: Add host flags for host compiler

This adds the host staging directory to the include path to make it use
the zlib.h files from the staging include directory and also link
against the zlib version from the staging directory.

This fixes a compile problem when the zlib header were not installed on
the build host.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[picked from openwrt-18.06]

package/boot/uboot-fritz4040/Makefile

@@ -46,8 +46,8 @@ export DTC
 
 define Build/Configure
 	$(Build/Configure/U-Boot)
-	$(HOSTCC) -o $(PKG_BUILD_DIR)/fritz/lzma2eva $(PKG_BUILD_DIR)/fritz/src/lzma2eva.c
-	$(HOSTCC) -o $(PKG_BUILD_DIR)/fritz/tichksum $(PKG_BUILD_DIR)/fritz/src/tichksum.c
+	$(HOSTCC) $(HOST_CFLAGS) $(HOST_LDFLAGS) -o $(PKG_BUILD_DIR)/fritz/lzma2eva $(PKG_BUILD_DIR)/fritz/src/lzma2eva.c -lz
+	$(HOSTCC) $(HOST_CFLAGS) $(HOST_LDFLAGS) -o $(PKG_BUILD_DIR)/fritz/tichksum $(PKG_BUILD_DIR)/fritz/src/tichksum.c
 	ln -sf $(STAGING_DIR_HOST)/bin/lzma $(PKG_BUILD_DIR)/fritz
 endef