libubox: update to Git HEAD (2025-11-14)

libubox: update to Git HEAD (2025-11-14)

a75209f62982 blobmsg: refactor blobmsg_cast_u64/s64
97d1839295e5 github: ci: minor fixes
e8dafede37fc github: ci: add powerpc arch
c1f0024fb052 github: ci: add cmake build and source directories
4dd874d2b412 github: ci: tests: rename test step
7686b12d11ba github: ci: tests: add build/bin to path
be21d12b2bed github: ci: disable json-c tests
af438ba0705f scripts: devel-build: disable json-c tests
240d2d5ef58b tests: shunit2: add test for _jshn_append leading space fix
82cb5fd66af9 libubox: Drop extraneous space when appending values to variable
17143f94dcbf libubox: add a simple build script
5b11fb18dc05 github: ci: add tests

https://github.com/openwrt/libubox/compare/e357be611cd9...a75209f62982

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>