On mingw, configure sets the name of the lxc symfile to
libvirt_lxc.defs rather than libvirt_lxc.syms.  But tarballs
must be arch-independent, regardless of the configure options
used for the tree where we ran 'make dist'.  This led to the
following failure in autobuild.sh:

  CCLD     libvirt-lxc.la
  CCLD     libvirt-qemu.la
/usr/lib64/gcc/i686-w64-mingw32/4.7.2/../../../../i686-w64-mingw32/bin/ld: cannot find libvirt_lxc.def: No such file or directory
collect2: error: ld returned 1 exit status
make[3]: *** [libvirt-lxc.la] Error 1
make[3]: *** Waiting for unfinished jobs....

We were already doing the right thing with libvirt_qemu.syms.

* src/Makefile.am (EXTRA_DIST): Don't ship a built file which
depends on configure for its final name.
Signed-off-by: NEric Blake <eblake@redhat.com>