Fix compile warnings in Driver.xst

[pali]

Perl.c: In function ‘XS_DBD__Perl__dr_data_sources’:
Perl.c:276:7: warning: variable ‘attr’ set but not used [-Wunused-but-set-variable]
SV * attr;
^~~~
Perl.c: In function ‘XS_DBD__Perl__db__login’:
Perl.c:320:7: warning: variable ‘attribs’ set but not used [-Wunused-but-set-variable]
SV * attribs;
^~~~~~~
Perl.c: In function ‘XS_DBD__Perl__db_last_insert_id’:
Perl.c:604:7: warning: variable ‘attr’ set but not used [-Wunused-but-set-variable]
SV * attr;
^~~~

[timbunce]

Thanks @pali