add review comment to sb files (#8824)

Context
Contributes to dotnet/source-build#3435

Adds comments to source-build files asking for the inclusion of the source-build team in PRs that alter SourceBuild* files. Non-reviewed changes could potentially cause issues down the line, be it in the downstream repos or the product build (as has happened in the past, see dotnet/source-build#3435 (comment))

Changes Made
added comments to source-build files asking for the inclusion of the source-build team in PRs that alter SourceBuild* file.

eng/SourceBuild.props

@@ -1,3 +1,5 @@
+<!-- Whenever altering this or other Source Build files, please include @dotnet/source-build-internal as a reviewer. -->
+
 <Project>
 
   <PropertyGroup>

eng/SourceBuildPrebuiltBaseline.xml

@@ -1,3 +1,6 @@
+<!-- Whenever altering this or other Source Build files, please include @dotnet/source-build-internal as a reviewer. -->
+<!-- See aka.ms/dotnet/prebuilts for guidance on what pre-builts are and how to eliminate them. -->
+
 <UsageData>
   <IgnorePatterns>
     <UsagePattern IdentityGlob="Microsoft.SourceBuild.Intermediate.*/*" />