[ci] Enable sonic-restapi build in PR validation. (#17397)

Why I did it
Enable sonic-restapi build in two platform to avoid build break on restapi target.

Work item tracking
Microsoft ADO (number only): 26048426
How I did it
How to verify it

azure-pipelines.yml

@@ -65,6 +65,7 @@ stages:
       - name: broadcom
         variables:
           swi_image: yes
+          INCLUDE_RESTAPI: y
       - name: mellanox
         variables:
           dbg_image: yes
@@ -79,6 +80,7 @@ stages:
         timeoutInMinutes: 1200
         variables:
           PLATFORM_ARCH: armhf
+          INCLUDE_RESTAPI: y
 
 - stage: Test
   dependsOn: BuildVS

rules/config

@@ -144,7 +144,7 @@ INCLUDE_MGMT_FRAMEWORK = y
 ENABLE_HOST_SERVICE_ON_START = y
 
 # INCLUDE_RESTAPI - build docker-sonic-restapi for configuring the switch using REST APIs
-INCLUDE_RESTAPI = n
+INCLUDE_RESTAPI ?= n
 
 # INCLUDE_NAT - build docker-nat for nat support
 INCLUDE_NAT = y