Identify non-local EXPORT_C_INCLUDE_DIRS in soong_to_convert

Soong only supports exporting include directories under the current
directory. So bring non-local directories up as a potential problem in
soong_to_convert.txt

Test: m -j $OUT/soong_to_convert.txt, inspect
Change-Id: I7a15b92e10a1d8b8d3496c6f0529a0d0824f301e