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: NEric Blake <eblake@redhat.com>