WaffleDefineCompilerFlags: FindPkgConfig variables are lists

In case of a list, cmake uses ';' as an element separator.
When the list is used as a string, as compiler CFLAGS,
the ';' ends up being picked up by the shell as an 'end of command'.

Iterate through the list and use ' ' as element separator.

Signed-off-by: Adrian Negreanu <adrian.negreanu@nxp.com>
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