third_party/stm32cube: Fix CORE_INIT visibility
third_party/stm32cube: Fix CORE_INIT visibility

The conditional that added the visibility was incorrectly placed
in the flags confguration rather than stm32cube_headers.

Change-Id: I2d2aea656bde2a36b6046a3501c0b99c176eb684
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/87382
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Commit-Queue: Brandon Kinman <ed0eaf5dad466b183f7a8162dd6f9f8d10d85144@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed