Fix IA64 GNU/Linux build

Fix IA64 GNU/Linux build

This commit should fix:

 ../../gdb/ia64-linux-nat.c: In function ‘void enable_watchpoints_in_psr(ptid_t)’:
 ../../gdb/ia64-linux-nat.c:535:56: error: no matching function for call to ‘get_thread_regcache(ptid_t&)’
    struct regcache *regcache = get_thread_regcache (ptid);
							 ^
 In file included from ../../gdb/ia64-linux-nat.c:25:0:
 ../../gdb/regcache.h:35:25: note: candidate: regcache* get_thread_regcache(process_stratum_target*, ptid_t)
  extern struct regcache *get_thread_regcache (process_stratum_target *target,
			  ^
 ../../gdb/regcache.h:35:25: note:   candidate expects 2 arguments, 1 provided
 ../../gdb/regcache.h:39:25: note: candidate: regcache* get_thread_regcache(thread_info*)
  extern struct regcache *get_thread_regcache (thread_info *thread);
			  ^
 ../../gdb/regcache.h:39:25: note:   no known conversion for argument 1 from ‘ptid_t’ to ‘thread_info*’

gdb/ChangeLog:
2020-05-16  Pedro Alves  <palves@redhat.com>

	* ia64-linux-nat.c
	(ia64_linux_nat_target) <enable_watchpoints_in_psr(ptid_t)>:
	Declare method.
	(enable_watchpoints_in_psr): Now a method of ia64_linux_nat_target.