[gdb] Fix gdb build on mips64-linux

When compiling for mips64-linux, we get:
...
src/gdb/linux-nat-trad.c:139:12: error: ‘gdbarch_num_regs’ was not declared \
  in this scope
  139 |   regnum < gdbarch_num_regs (regcache->arch ());
...

Fix this by including gdbarch.h in linux-nat-trad.c, similar to commit
b1c896b365 "Fix gdb build on macOS".

Build on mips64-linux.

gdb/ChangeLog:

2019-08-15  Tom de Vries  <tdevries@suse.de>

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

---
 gdb/linux-nat-trad.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/gdb/linux-nat-trad.c b/gdb/linux-nat-trad.c
index 04507e8041..c6208dccdb 100644
--- a/gdb/linux-nat-trad.c
+++ b/gdb/linux-nat-trad.c
@@ -22,6 +22,7 @@
 
 #include "nat/gdb_ptrace.h"
 #include "inf-ptrace.h"
+#include "gdbarch.h"
 
 /* Fetch register REGNUM from the inferior.  */