Export header check allows WholeStaticLibs

cc/linker.go code assumes that export_static_lib_headers can apply
regardless of whether a library is a regular or 'whole' static lib.
However, this check prevents that code from working.

Bug: 183654927
Test: use export_static_lib_headers like this
Change-Id: I680f90514e41224a7077630167ab4e481909d89c