meson.build: Re-enable KVM support for MIPS

After converting from configure to meson, KVM support is lost for MIPS,
so re-enable it in meson.build.

Fixes: fdb75aef ("configure: remove target configuration")
Fixes: 8a19980e ("configure: move accelerator logic to meson")
Cc: aolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Message-Id: <1602059975-10115-3-git-send-email-chenhc@lemote.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>