Predefine U_IN_DOXYGEN in NDK Doxyfile

U_IN_DOXYGEN is used in various ICU4C headers to show the correct
APIs.

Bug: 168242257
Test: Resolved some warnings produced from development/tools/ndk/build_ndk_docs.py
Change-Id: Idc6c84a32c614d4de32672675cb6c570143b8264
1 file changed