Fix freetype cmake config files

[tronical]

The way of including freetype headers is by using

#include <ft2build.h>
#include FT_FREETYPE_H

along with $root/include/freetype which has $root/include/freetype/ft2build.h
and then the rest of the headers under $root/include/freetype/freetype2/*.h.

The portfile.cmake moves the headers one directory up into $root/include
so that there's $root/include/ft2build.h and the rest of the headers
under $root/include/freetype/.

This in turn means that the above way of including header files requires
$root/include to be in the include search path. That's not the case by
default and the generated freetype-config.cmake also doesn't include
that.

Therefore the freetype config approach results in failing builds when
applications wants to use it. This is easy to reproduce using

find_package(Freetype CONFIG REQUIRED)
...
target_link_libraries(foo PRIVATE freetype)

As a remedy, this patch fixes the freetype-config.cmake file to add
$root/include in the include search path.

[dan-shaw]

Thanks for the PR!