On a mingw build, 'make distcheck' fails with:

  GEN      libvirt_qemu.def
make[3]: *** No rule to make target `../../src/libvirt_lxc.syms', needed by `libvirt_lxc.def'.  Stop.

I traced it to a missing entry in EXTRA_DIST.  But rather than keep
the entire list in sync, it is easier to list the three syms files
that drive .so files directly, and then reuse existing makefile
variables for the remaining files (that is, I validated that all
remaining files are added to SYM_FILES, possibly via USED_SYM_FILES,
according to makefile conditionals).

Problem introduced in commit 3d1596b0 (v1.0.2).

* src/Makefile.am (EXTRA_DIST): Ensure all syms files are shipped.
Signed-off-by: NEric Blake <eblake@redhat.com>