stubs: Move qemu_fd_register stub to util/main-loop.c

stubs: Move qemu_fd_register stub to util/main-loop.c

The linker of MinGW sometimes runs into the following problem:

libqemuutil.a(util_main-loop.c.obj): In function `qemu_fd_register':
/builds/huth/qemu/build/../util/main-loop.c:331: multiple definition of
 `qemu_fd_register'
libqemuutil.a(stubs_fd-register.c.obj):/builds/huth/qemu/stubs/fd-register.c:5:
 first defined here
collect2: error: ld returned 1 exit status
/builds/huth/qemu/rules.mak:88: recipe for target 'tests/test-timed-average.exe'
 failed

qemu_fd_register() is defined in util/main-loop.c for WIN32, so let's simply
move the stub also there in the #else part of the corresponding #ifndef
to fix this problem.

Message-Id: <20200903054503.425435-1-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>