Add missing unspecified dependency on solver

Author: Redth

constraint-layout/nuget/Xamarin.Android.Support.Constraint.Layout.nuspec

@@ -12,6 +12,9 @@
     <projectUrl>https://github.com/xamarin/AndroidSupportComponents/</projectUrl>
     <licenseUrl>https://github.com/xamarin/AndroidSupportComponents/blob/master/LICENSE.md</licenseUrl>
     <iconUrl>https://raw.githubusercontent.com/xamarin/GooglePlayServicesComponents/master/icons/constraint-layout_128x128.png</iconUrl>
+    <dependencies>
+      <dependency id="Xamarin.Android.Support.Constraint.Layout.Solver" version="[$version$]"/>
+    </dependencies>
   </metadata>
   <files>
     <file src="output/Xamarin.Android.Support.Constraint.Layout.dll" target="lib/MonoAndroid70" />