fontcache.bbclass: avoid native recipes depending on target fontconfig

fontcache.bbclass: avoid native recipes depending on target fontconfig

* this caused liberation-font-native to depend on TUNE_PKGARCH target fontconfig
  because ${MLPREFIX}fontconfig-utils is added to RDEPENDS in anonymous python

* the dependency tree for liberation-font-native got much shorter
  (just quilt-native and liberation-font-native itself):
   2 after/pn-buildlist
  78 before/pn-buildlist

* fixes graphviz-native signature issue as well as detected with sstate-diff-machines.sh

  $ bitbake-diffsigs \
    sstate-after/mako/x86_64-linux/graphviz-native/8.1.0.do_populate_sysroot.sigdata.184d4fd355f1e7a2d7d929ef4b5f62b94e2071df9dd674b2067ec21bfc7bcc1b \
    sstate-after/qemux86-64/x86_64-linux/graphviz-native/8.1.0.do_populate_sysroot.sigdata.35da674d2dbc275bac02869dfce4165466315023910bdef65a6026e2cb942a46
  Hash for task dependency liberation-fonts-native:do_populate_sysroot changed from 310d3da04ad9abf8ee99997e53d1ffa71c2b7d9d60fb0e8de85171a9ab6a77d3 to 048420ad1410c2b8d95498fc3c11681207335a2c722836f1f3e683cc449814da

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>