Native: mention the way to suppress Xcode version check in the build

When building Kotlin/Native from sources on macOS, build scripts check
that installed Xcode has specific major version. It is possible to
suppress the version mismatch error (with caution), but the error
message didn't mention that.

Fix that by adding a hint to the error message.

kotlin-native/build-tools/src/main/kotlin/org/jetbrains/kotlin/PlatformInfo.kt

@@ -65,6 +65,7 @@ object PlatformInfo {
             if (currentMajorVersion != requiredMajorVersion) {
                 throw IllegalStateException(
                         "Incorrect Xcode version: ${currentXcodeVersion}. Required major Xcode version is ${requiredMajorVersion}."
+                                + "\nYou can try '-PcheckXcodeVersion=false' to suppress this error, the result might be wrong."
                 )
             }
         }