FPHSA: Fix empty version message in exact match

`find_package_check_version()` would use variable `_FOUND_VERSION` set
by `find_package_handle_standard_args()` in a single branch when doing
exact matching, causing empty version messages if it was instead called
directly.

This was missed in commit 6bfc442fde (FPHSA: add support of version
range, 2020-09-22, v3.19.0-rc1~122^2~1) when the version check was moved
into its own function.