LOG4NET-540 add empty dependency groups for all supported target frameworks
diff --git a/log4net.nuspec b/log4net.nuspec
index 3a0b42a..27fac16 100644
--- a/log4net.nuspec
+++ b/log4net.nuspec
@@ -39,6 +39,12 @@
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <tags>logging log tracing logfiles</tags>
     <dependencies>
+      <group targetFramework="net20-full" />
+      <group targetFramework="net35-full" />
+      <group targetFramework="net40-full" />
+      <group targetFramework="net45-full" />
+      <group targetFramework="net35-client" />
+      <group targetFramework="net40-client" />
       <group targetFramework=".NETStandard1.3">
         <dependency id="System.AppContext" version="[4.1.0, )" />
         <dependency id="System.Collections.NonGeneric" version="[4.0.1, )" />