perl.h: move PERL_GET_CONTEXT fallbacks earlier

A recent commit added a 'dTHX' to inline.h for the first time;
this broke -DPERL_GLOBAL_STRUCT* builds.

The fix is to move the 'fallback' definitions in perl.h, such as

    #ifndef PERL_GET_CONTEXT
    #  define PERL_GET_CONTEXT             PERL_GET_INTERP

to earlier in the file so that they come before

    #include "inline.h"