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
diff --git a/docs/Doxyfile b/docs/Doxyfile
index a1bd960..ea22337 100644
--- a/docs/Doxyfile
+++ b/docs/Doxyfile
@@ -1638,6 +1638,7 @@
     "__attribute__(x)=" \
     __ANDROID__ \
     __BIONIC__ \
+    "U_IN_DOXYGEN=1" \ # Required by the ICU4C module only
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then
 # this tag can be used to specify a list of macro names that should be expanded.