squashfs: fix build when zlib isn't available on the host

Pass CFLAGS and LDFLAGS so that when building squashfs, it finds the
zlib installed in $(HOST_DIR).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/squashfs/squashfs.mk

@@ -18,7 +18,7 @@ define SQUASHFS_INSTALL_TARGET_CMDS
 endef
 
 define HOST_SQUASHFS_BUILD_CMDS
- $(HOST_MAKE_ENV) $(MAKE) -C $(@D)/squashfs-tools/
+ $(HOST_MAKE_ENV) $(MAKE) CFLAGS="$(HOST_CFLAGS)" LDFLAGS="$(HOST_LDFLAGS)" -C $(@D)/squashfs-tools/
 endef
 
 define HOST_SQUASHFS_INSTALL_CMDS