libnice: create ${S}/m4 directory before gtkdocize is called

* otherwise it fails with:
+ gtkdocize --docdir /OE/build/shr-core/tmp-eglibc/work/core2-64-oe-linux/libnice/0.0.13-r4/libnice-0.0.13
  ln: target 'm4/' is not a directory: No such file or directory
  cp: cannot create regular file 'm4/': Not a directory

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>