Revert "Temporary remove all -Werror flags"

This reverts commit 89c09214eeed951684405c10c233304834a89beb.

kotlin-native/backend.native/build.gradle

@@ -56,7 +56,7 @@ compileCompilerKotlin {
     // but not for Kotlin.
     dependsOn('generateCompilerProto')
     kotlinOptions.jvmTarget = "1.8"
-//    kotlinOptions.allWarningsAsErrors=true
+    kotlinOptions.allWarningsAsErrors=true
     kotlinOptions.freeCompilerArgs += ['-Xopt-in=kotlin.RequiresOptIn', '-Xopt-in=org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI', '-Xskip-prerelease-check']
 }
 
@@ -323,4 +323,4 @@ publishing {
             from components.java
         }
     }
-}
+}