Include gdbarch.h in m68k-linux-nat.c

Include gdbarch.h in m68k-linux-nat.c

Fix this compilation error, and a bunch of similar ones:

      CXX    m68k-linux-nat.o
    /home/smarchi/src/binutils-gdb/gdb/m68k-linux-nat.c: In function ‘void fetch_register(regcache*, int)’:
    /home/smarchi/src/binutils-gdb/gdb/m68k-linux-nat.c:133:9: error: ‘gdbarch_register_name’ was not declared in this scope
             gdbarch_register_name (gdbarch, regno),
             ^~~~~~~~~~~~~~~~~~~~~

gdb/ChangeLog:

	* m68k-linux-nat.c: Include gdbarch.h.

Change-Id: I7cd47bc5d094241b2596e29c244eb55ed11f7a02