build: avoid $(builddir) in Makefile

build: avoid $(builddir) in Makefile

Older automake 1.9.6 (hello there, RHEL 5) did not populate
$(builddir), which meant 'make check' failed with:

make[3]: *** No rule to make target `/.libs/libvirt.la', needed by `check-symfile'.  Stop.

For that matter, even newer automake doesn't directly emit rules
to build .libs/libvirt.la; we are better off basing our rules
on the public ./libvirt.la.

* src/Makefile.am (check-symfile): Delete useless variable.