regression: hide include-not-found errors during library discovery

This regression was made during a refactoring of the Arduino preprocessor.
In particular the wrong change was part of this commit:
arduino@0585435#diff-65ff16cbee816c0f443157444d99bcc144beee06c3329aec891894c8aeda7b27L372-R378

-                       preproc_stderr, preproc_err = GCCPreprocRunner(ctx, sourcePath, targetFilePath, includes)
+                       var preproc_stdout []byte
+                       preproc_stdout, preproc_stderr, preproc_err = preprocessor.GCC(sourcePath, targetFilePath, includes, ctx.BuildProperties)
+                       if ctx.Verbose {
+                               ctx.WriteStdout(preproc_stdout)
+                               ctx.WriteStdout(preproc_stderr)
+                       }

Previously GCCPreprocRunner, in verbose modem will show ONLY the stdout of
the process, instead the "refactored" code wrongly added also stderr to the
output.

For reference this is the old GCCPreprocRunner implementation:
arduino@0585435#diff-371f93465ca5a66f01cbe876348f67990750091d27a827781c8633456b93ef3bL36

-func GCCPreprocRunner(ctx *types.Context, sourceFilePath *paths.Path, targetFilePath *paths.Path, includes paths.PathList) ([]byte, error) {
-       cmd, err := prepareGCCPreprocRecipeProperties(ctx, sourceFilePath, targetFilePath, includes)
-       if err != nil {
-               return nil, err
-       }
-       _, stderr, err := utils.ExecCommand(ctx, cmd, utils.ShowIfVerbose /* stdout */, utils.Capture /* stderr */)
-       return stderr, err
-}

This commit fixes the regression.

legacy/builder/container_find_includes.go

@@ -374,7 +374,6 @@ func findIncludesUntilDone(ctx *types.Context, cache *includeCache, sourceFileQu
 			preprocStdout, preprocStderr, preprocErr = preprocessor.GCC(sourcePath, targetFilePath, includeFolders, ctx.BuildProperties)
 			if ctx.Verbose {
 				ctx.WriteStdout(preprocStdout)
-				ctx.WriteStdout(preprocStderr)
 			}
 			// Unwrap error and see if it is an ExitError.
 			_, isExitErr := errors.Cause(preprocErr).(*exec.ExitError)