gdb: Fix native build on Linux/Alpha.

gdb: Fix native build on Linux/Alpha.

Fixes:

  CXX    alpha-linux-nat.o
alpha-linux-nat.c: In member function 'virtual CORE_ADDR alpha_linux_nat_target::register_u_offset(gdbarch*, int, int)':
alpha-linux-nat.c:93:16: error: 'gdbarch_pc_regnum' was not declared in this scope
   if (regno == gdbarch_pc_regnum (gdbarch))
                ^~~~~~~~~~~~~~~~~
alpha-linux-nat.c:93:16: note: suggested alternative: 'gdbarch_info'
   if (regno == gdbarch_pc_regnum (gdbarch))
                ^~~~~~~~~~~~~~~~~
                gdbarch_info
alpha-linux-nat.c:97:15: error: 'gdbarch_fp0_regnum' was not declared in this scope
   if (regno < gdbarch_fp0_regnum (gdbarch))
               ^~~~~~~~~~~~~~~~~~
alpha-linux-nat.c:97:15: note: suggested alternative: 'gdbarch_info'
   if (regno < gdbarch_fp0_regnum (gdbarch))
               ^~~~~~~~~~~~~~~~~~
               gdbarch_info
make[2]: *** [Makefile:1638: alpha-linux-nat.o] Error 1
make[2]: Leaving directory '/home/chris/tmp/binutils-gdb/gdb'

2019-09-12  Christian Groessler  <chris@groessler.org>

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