build: fix another virdbus issue on mingw

build: fix another virdbus issue on mingw

Depending on the set of mingw packages installed, it is possible
that other .c files hit the mingw header pollution from the
virdbus.h file.

In file included from ../../src/rpc/virnetserver.c:39:0:
../../src/util/virdbus.h:41:35: error: expected ';', ',' or ')' before 'struct'
                       const char *interface,
                                   ^

* src/util/virdbus.h (virDBusCallMethod): Match .c file change.

Signed-off-by: Eric Blake <eblake@redhat.com>