libxml: Remove -Werror=array-bounds from compiler flags

libxml uses zero-length array tricks causing array-bounds warnings.

The warnings have been verified to be harmless in 2.9.10; for extra
security, this should be re-checked after updating.

Removing -Werror=array-bounds here allows us to use -Werror=array-bounds
globally in OPTIMIZE_FOR=security mode.

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>