KT-45777: Add -dontwarn com.google.gson.** to Proguard rules

The previous commit added gson to `incremental-compilation-impl` module.
However, gson is currently not added to -libraryjars for Proguard, so
running with -Pteamcity=true would fail with
```
> Task :kotlin-compiler:proguard
Warning: ... can't find referenced class com.google.gson.Gson
Warning: ... can't find referenced class com.google.gson.GsonBuilder
   You may need to add missing library jars or update their versions.
   If your code works fine without the missing classes, you can suppress
   the warnings with '-dontwarn' options.
   (http://proguard.sourceforge.net/manual/troubleshooting.html#unresolvedclass)
```

This commits adds -dontwarn com.google.gson.** to work around this
error. The proper fix is probably to add gson (and many other libraries)
to Proguard's -libraryjars.

prepare/compiler/compiler.pro

@@ -38,6 +38,7 @@
 -dontwarn org.jline.builtins.Nano$Buffer
 -dontwarn com.intellij.util.io.TarUtil
 -dontwarn com.intellij.util.io.Compressor$Tar
+-dontwarn com.google.gson.**
 
 # Some annotations from intellijCore/annotations.jar are not presented in org.jetbrains.annotations
 -dontwarn org.jetbrains.annotations.*