Fix build error in aix-thread.c

Fix build error in aix-thread.c

Obvious fix for:

aix-thread.c: In function 'char* pd_status2str(int)':
aix-thread.c:163:33: error: deprecated conversion from string constant to 'char*' [-Werror=write-strings]
     case PTHDB_SUCCESS:  return "SUCCESS";
                                 ^
gdb/ChangeLog:

	* aix-thread.c (pd_status2str): Change return type to const char *.