Fix mixin annotation processor issues

These issues were caused by CompilationConfig.configureCompilation()
overwriting JavaCompile's options.compilerArgs list which removed any
previously added arguments, including those added by fabric-loom and
mixingradle.

buildSrc/src/main/kotlin/com/dfsek/terra/CompilationConfig.kt

@@ -19,7 +19,7 @@ fun Project.configureCompilation() {
     tasks.withType<JavaCompile> {
         options.encoding = "UTF-8"
         doFirst {
-            options.compilerArgs = mutableListOf("-Xlint:all")
+            options.compilerArgs.add("-Xlint:all")
         }
     }