[gdb/build, s390x] Fix build after gdbarch_tdep changes

[gdb/build, s390x] Fix build after gdbarch_tdep changes

Commit 345bd07cce33 ("gdb: fix gdbarch_tdep ODR violation") changes a
declaration in s390-tdep.h from

   struct gdbarch_tdep { ... };

to

   struct s390_gdbarch_tdep : gdbarch_tdep { ... };

and now requires that gdbarch_tdep has been declared before.  Which is
usually the case, except when compiling s390-linux-nat.c, where
s390-tdep.h is included before gdbarch.h.  Thus the s390x build errors out
with the compiler complaining about a missing class name after the colon.

Fix this in s390-linux-nat.c, by including gdbarch.h before s390-tdep.h.