gdb: Change a VEC to std::vector in btrace.{c,h}

gdb: Change a VEC to std::vector in btrace.{c,h}

Replace a VEC with a std::vector in btrace.h, and update btrace.c to
match.  It is worth noting that this code appears to be currently
untested by the GDB testsuite.  I've tried to do a like for like
replacement when moving to std::vector, with minimal refactoring to
try and avoid introducing any bugs.

As the new vector is inside a union I've currently used a pointer to
vector, which makes the code slightly uglier than it might otherwise
be, but again, due to lack of testing I'm reluctant to start
refactoring the code in a big way.

gdb/ChangeLog:

	* btrace.c (btrace_maint_clear): Update to handle change from VEC
	to std::vector.
	(btrace_maint_decode_pt): Likewise, and move allocation of the
	vector outside of the loop.
	(btrace_maint_update_packets): Update to handle change from VEC to
	std::vector.
	(btrace_maint_print_packets): Likewise.
	(maint_info_btrace_cmd): Likewise.
	* btrace.h: Remove use of DEF_VEC_O.
	(typedef btrace_pt_packet_s): Delete.
	(struct btrace_maint_info) <packets>: Change fromm VEC to
	std::vector.
	* gdbsupport/btrace-common.h: Remove 'vec.h' include.