Add plain option for target coverage

[rlalik]

When no visibility is specified for target, adding coverage with any of the two options: PUBLIC or INTERFACE (PRIVATE is default) results in error:

The plain signature for target_link_libraries has already been used with
the target "xxxxx".  All uses of target_link_libraries with a
target must be either all-keyword or all-plain.

where

target_link_libraries(xxxxx libA libB) # no visibility specified

I added PLAIN option which do not add any visibility, as the default option is always PRIVATE.

Alternatively, one should add PRIVATE option and leave no option equivalent to PLAIN but this will break back-compatibility, which might not be an issue perhaps. Actually this is what I would prefer Makes it more consistent.