Long explanation for small commit:

With the recent reorganization of #includes, things get a little bit
fragile: When gcc with -O is used, glibc's <stdio.h> defines a few
inline functions (see <bits/stdio.h>), and "Stg.h" defines some global
variables which reside in registers. But the latter must happen
*before* any function definition has been seen, otherwise the
generated code could be invalid. Consequently gcc complains like:

   In file included from Stg.h:182,
                    from mkNativeHdr.c:12:
   Regs.h:293: global register variable follows a function definition
   Regs.h:302: global register variable follows a function definition

The solution is quite simple: Always #include "Stg.h" *before* any
system headers.