scripts/gdb: fix lx-version string output

A bug is present in GDB which causes early string termination when
parsing variables.  This has been reported [0], but we should ensure
that we can support at least basic printing of the core kernel strings.

For current gdb version (has been tested with 7.3 and 8.1), 'lx-version'
only prints one character.

  (gdb) lx-version
  L(gdb)

This can be fixed by casting 'linux_banner' as (char *).

  (gdb) lx-version
  Linux version 4.19.0-rc1+ (changbin@acer) (gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)) #21 SMP Sat Sep 1 21:43:30 CST 2018

[0] https://sourceware.org/bugzilla/show_bug.cgi?id=20077

[[email protected]: add detail to commit message]
Link: http://lkml.kernel.org/r/[email protected]
Fixes: 2d061d9 ("scripts/gdb: add version command")
Signed-off-by: Du Changbin <[email protected]>
Signed-off-by: Kieran Bingham <[email protected]>
Acked-by: Jan Kiszka <[email protected]>
Cc: Jan Kiszka <[email protected]>
Cc: Jason Wessel <[email protected]>
Cc: Daniel Thompson <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>