Silence some warnings

```ppport.h``` should be included after ```XSUB.h```, otherwise (atleast on OSX you get this):

```
In file included from Stash.xs:40:
/Users/jacques/perl/lib/darwin/CORE/XSUB.h:158:9: warning: 'dAX' macro redefined
#define dAX const I32 ax = (I32)(MARK - PL_stack_base + 1)
        ^
./ppport.h:4073:11: note: previous definition is here
#  define dAX                            I32 ax = MARK - PL_stack_base + 1
          ^
In file included from Stash.xs:40:
/Users/jacques/perl/lib/darwin/CORE/XSUB.h:160:9: warning: 'dAXMARK' macro redefined
#define dAXMARK                         \
        ^
./ppport.h:4083:11: note: previous definition is here
#  define dAXMARK                        I32 ax = POPMARK; \
          ^
In file included from Stash.xs:40:
/Users/jacques/perl/lib/darwin/CORE/XSUB.h:164:9: warning: 'dITEMS' macro redefined
#define dITEMS I32 items = (I32)(SP - MARK)
        ^
./ppport.h:4077:11: note: previous definition is here
#  define dITEMS                         I32 items = SP - MARK
          ^
In file included from Stash.xs:40:
/Users/jacques/perl/lib/darwin/CORE/XSUB.h:175:9: warning: 'dXSTARG' macro redefined
#define dXSTARG SV * const targ = ((PL_op->op_private & OPpENTERSUB_HASTARG) \
        ^
./ppport.h:4080:11: note: previous definition is here
#  define dXSTARG                        SV * targ = sv_newmortal()
          ^
In file included from Stash.xs:40:
/Users/jacques/perl/lib/darwin/CORE/XSUB.h:198:9: warning: 'UNDERBAR' macro redefined
#define UNDERBAR  find_rundefsv()
        ^
./ppport.h:4070:11: note: previous definition is here
#  define UNDERBAR                       DEFSV
          ^
Stash.xs:156:13: warning: 15 enumeration values not handled in switch: 'SVt_NULL', 'SVt_BIND', 'SVt_IV'... [-Wswitch]
    switch (SvTYPE(SvRV(root))) {
            ^
/Users/jacques/perl/lib/darwin/CORE/sv.h:288:20: note: expanded from macro 'SvTYPE'
#define SvTYPE(sv)      ((svtype)((sv)->sv_flags & SVTYPEMASK))
                        ^
6 warnings generated.
```

Also added a ```default``` to one of the ```switch()``` statements.