automake: explicitly set TARGET_RADEON_{WINSYS,COMMON}

automake: explicitly set TARGET_RADEON_{WINSYS,COMMON}

Originally the variables were set only once via the ?= operator but
that causes issues when doing incremental builds. They appear to be
undefined and missing from the dependency list despite their addition
to LIBADD.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84807
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>