Renamed objc klib in the sample to objective_c to eliminate naming

conflict woth objc platform lib.
Made objc sample compile with the platform libs.

samples/objc/build.gradle

@@ -1,15 +1,14 @@
 apply plugin: 'konan'
 
 konanInterop {
-    objc {
+    objective_c {
         target "macbook"
     }
 }
 
-
 konanArtifacts {
     Window {
-        useInterop 'objc'
+        useInterop 'objective_c'
         target "macbook"
     }
 }

samples/objc/src/main/kotlin/Window.kt

@@ -1,4 +1,6 @@
-import objc.*
+import platform.objc.*
+import platform.osx.*
+import objective_c.*
 import kotlinx.cinterop.*
 
 fun main(args: Array<String>) {