feature.h: use the correct format for U32 for cop_feature_bits

[tonycoz]

Fixes from 32-bit builds:

In file included from dump.c:31:0:
dump.c: In function 'S_do_op_dump_bar':
feature.h:751:29: warning: format '%x' expects argument of type 'unsigned int', but argument 3 has type 'U32 {aka long unsigned int}' [-Wformat=]
         PerlIO_printf(file, "0x%08x", cop->cop_features.bits[0]); \
                             ^
feature.h:751:29: note: in definition of macro 'DUMP_FEATURE_BITS'
         PerlIO_printf(file, "0x%08x", cop->cop_features.bits[0]); \
                             ^~~~~~~~

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• This set of changes does not require a perldelta entry.

[richardleach]

LGTM