gdb: initialize the data_head variable to eliminate compilation warnings

gdb: initialize the data_head variable to eliminate compilation warnings

On a machine with gcc 12, I get this warning:

  CXX    nat/linux-btrace.o
In function ‘btrace_error linux_read_bts(btrace_data_bts*, btrace_target_info*, btrace_read_type)’,
    inlined from ‘btrace_error linux_read_btrace(btrace_data*, btrace_target_info*, btrace_read_type)’ at ../gdb/nat/linux-btrace.c:935:29:
../gdb/nat/linux-btrace.c:865:21: warning: ‘data_head’ may be used uninitialized [-Wmaybe-uninitialized]
  865 |   pevent->last_head = data_head;
      |   ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
../gdb/nat/linux-btrace.c: In function ‘btrace_error linux_read_btrace(btrace_data*, btrace_target_info*, btrace_read_type)’:
../gdb/nat/linux-btrace.c:792:9: note: ‘data_head’ was declared here
  792 |   __u64 data_head, data_tail;
      |         ^~~~~~~~~

Fix this by initializing the 'data_head' variable.

Tested by rebuilding on x86_64 openSUSE Tumbleweed with gcc 12.