grub: fix load module all_video failed

grub: fix load module all_video failed

While using oe-core toolchain to strip grub module 'all_video.mod',
it stripped symbol table:
--------------
root@localhost:~# objdump -t all_video.mod

all_video.mod:     file format elf64-x86-64

SYMBOL TABLE:
no symbols
--------------

It caused grub to load module all_video failed.
(This module will be loaded by defalut which configed in grub.cfg)
--------------
grub> insmod all_video
error: no symbol table.
--------------

Tweak strip option to keep symbol .module_license could workaround
the issue.
--------------
root@localhost:~# objdump -t all_video.mod

all_video.mod:     file format elf64-x86-64

SYMBOL TABLE:
0000000000000000 l    d  .text  0000000000000000 .text
0000000000000000 l    d  .data  0000000000000000 .data
0000000000000000 l    d  .module_license        0000000000000000 .module_license
0000000000000000 l    d  .bss   0000000000000000 .bss
0000000000000000 l    d  .moddeps       0000000000000000 .moddeps
0000000000000000 l    d  .modname       0000000000000000 .modname
--------------

(From OE-Core rev: 17e7eb96e5446821ad81977ac9ccac26b05e67a7)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>