Enable Prometheus mtail build for ARM64

arm64 binaries are available currently.

Change-Id: Iabf606848a6160eaf9e2494a2564f37c6db09882
Signed-off-by: Bartosz Bezak <[email protected]>
(cherry picked from commit a7ada21)

Author: bbezak

kolla/image/unbuildable.py

@@ -17,7 +17,6 @@
 UNBUILDABLE_IMAGES = {
     'aarch64': {
         "bifrost-base",        # someone need to get upstream working first
-        "prometheus-mtail",    # no aarch64 binary
     },
 
     # Issues for SHA1 keys: