freetype: host-freetype-patch requires host-automake

The "make host-freetype-patch" fails because using the autogen.sh script
doesn't find the autoconf tool. The problem was already fixed for
"make freetype-patch" adding the "host-automake" as a prerequisite to
freetype-patch, so fix it by adding "host-automake" as a prerequisite to
"host-freetype-patch" too.

Fix build failure:
running `autoconf --force'
./autogen.sh: line 15: autoconf: command not found
error while running `autoconf --force'
package/pkg-generic.mk:146: recipe for target 'buildroot/output/build/host-freetype-2.5.5/.stamp_patched' failed

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>