install: Fix regression when using default destinations

In commit 9fc20a4f (install: Add sane set of defaults for DESTINATION
and file type parameters, 2018-11-02, v3.14.0-rc1~410^2~1), a regression
was introduced, in which an `install(TARGETS)` with a
RUNTIME/LIBRARY/ARCHIVE DESTINATION but no PUBLIC_HEADER/PRIVATE_HEADER
DESTINATION would then install the headers.  The old behavior did not do
this.  Restore the old behavior.

Fixes: #20326