Fix build breakage when libipt is available

Fix build breakage when libipt is available

Fix build regression introduced by 0860c437cbe4 ("btrace: Store
btrace_insn in an std::vector"):

  src/gdb/btrace.c: In function ‘void ftrace_add_pt(btrace_thread_info*, pt_insn_decoder*, int*, std::vector<unsigned int>&)’:
  src/gdb/btrace.c:1329:38: error: invalid initialization of reference of type ‘const btrace_insn&’ from expression of type ‘btrace_insn*’
      ftrace_update_insns (bfun, &btinsn);
					^
  src/gdb/btrace.c:648:1: note: in passing argument 2 of ‘void ftrace_update_insns(btrace_function*, const btrace_insn&)’
   ftrace_update_insns (struct btrace_function *bfun, const btrace_insn &insn)
   ^

gdb/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* btrace.c (ftrace_add_pt): Pass btrace_insn to
	ftrace_update_insns by reference instead of pointer.