btrace: fix build fail with 32-bit BFD

btrace: fix build fail with 32-bit BFD

When compiling GDB with 32-bit BFD, the build fails with:

In file included from btrace.h:33:0,
                 from btrace.c:23:
/usr/include/intel-pt.h:1643:51: note: expected 'int (*)(uint8_t *, size_t,
 const struct pt_asid *, uint64_t, void *)' but argument is of type 'int
 (*)(gdb_byte *, size_t, const struct pt_asid *, CORE_ADDR, void *)' extern
 pt_export int pt_image_set_callback(struct pt_image *image, ^

gdb/
	* btrace.c (btrace_pt_readmem_callback): Change type of PC argument.