libexpat: do not install soname symlink

The .so symlinks (without any suffix) are only used by compiler/linker and
not needed on the target system.
See https://github.com/openwrt/packages/pull/274 for a short discussion.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Merge pull request #281 from mhei/libs/expat

libexpat: do not install soname symlink