Set INCLUDE_UNI_CORE_DATA=1 when rebuilding ICU4C time zone data.

This is now necessary to build .dat files that are useable for ICU4J as
well, and will need to be used for any future updates of the .dat file:

https://android.googlesource.com/platform/external/icu/+/6b52738

Change-Id: Ifa3a7469a66ab932db20876697e45814f0c5ddc3

libc/tools/zoneinfo/update-tzdata.py

@@ -140,7 +140,7 @@ def BuildIcuToolsAndData(data_filename):
 
   # Regenerate the .dat file.
   os.chdir(icu_working_dir)
-  subprocess.check_call(['make', '-j32'])
+  subprocess.check_call(['make', 'INCLUDE_UNI_CORE_DATA=1', '-j32'])
 
   # Copy the .dat file to its ultimate destination.
   icu_dat_data_dir = '%s/stubdata' % icu_dir