Fix installing from read-only ${.OBJDIR}

Fix installing from read-only ${.OBJDIR}

By default, install command preserves mtime, and files in the source tree
always have older mtime than a file created by touch command, in this case
${.OBJDIR}/openssl/.dummy, even though it's created earlier by dependency.
Since .dummy is newer, header files get created in objdir at install target
and that requires write permission on ${.OBJDIR}.