lib: make LZ4_decompress_safe_forceExtDict() static

LZ4_decompress_safe_forceExtDict() is only used in
lib/lz4/lz4_decompress.c, make it static to fix the build warning about
"no previous prototype" [1].

[1] https://lore.kernel.org/lkml/202206260948.akgsho1q-lkp@intel.com/

Link: https://lkml.kernel.org/r/1656298965-8698-1-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>