gdb: include gdbarch.h in hppa-linux-nat.c

hppa-linux-nat.c fails to build due to the gdbarch stuff not being
declared, for example:

    hppa-linux-nat.c: In function ‘void fetch_register(regcache*, int)’:
    hppa-linux-nat.c:230:7: error: ‘gdbarch_cannot_fetch_register’ was not declared in this scope
       if (gdbarch_cannot_fetch_register (gdbarch, regno))

Include gdbarch.h to fix it.

gdb/ChangeLog:

	PR gdb/25045
	* hppa-linux-nat.c: Include gdbarch.h.