Add missing declaration for reflection build

Author: josesimoes

nanoFramework.CoreLibrary/CoreLibrary.nfproj

@@ -29,6 +29,7 @@
     <Deterministic>true</Deterministic>
     <IncludeSymbols>true</IncludeSymbols>
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+    <DefineConstants>$(DefineConstants);NANOCLR_REFLECTION;</DefineConstants>
     <ContinuousIntegrationBuild Condition="'$(TF_BUILD)' == 'true'">True</ContinuousIntegrationBuild>
     <AllowedOutputExtensionsInPackageBuildOutputFolder>$(AllowedOutputExtensionsInPackageBuildOutputFolder);.pdb</AllowedOutputExtensionsInPackageBuildOutputFolder>
   </PropertyGroup>