ICU-21294 Fix install with --with-data-packaging=files

[dlaugt]

In Linux, the installation via make install didn't work because some directories weren't created.

The Makefile in source/data contains the code to create the directories:

ifeq ($(PKGDATA_MODE),files)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(CURR_TREE)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(LANG_TREE)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(REGION_TREE)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(ZONE_TREE)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(UNIT_TREE)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(BREAK_TREE)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(COLLATION_TREE)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(RBNF_TREE)
	$(MKINSTALLDIRS) $(ICUPKGDATA_DIR)/$(ICUDATA_NAME)/$(TRANSLIT_TREE)
endif

The problem is the variables $(CURR_TREE), $(LANG_TREE), ..., $(TRANSLIT_TREE) were not defined anymore in the Makefile. They have been suppressed with the commit ee9c0b9

With the redefinition of those variables, the make install works again when ./configure has been launched with option: --with-data-packaging=files.

Checklist

• [x] Required: Issue filed: ICU-21294
• [x] Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• [x] Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• [ ] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

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