lgpio: Fix cross-compilation build
The upstream Makefile [1] supports the CROSS_PREFIX variable to specify
the cross-compiler toolchain prefix. Without this, the build fails
when cross-compiling (e.g., for aarch64-multiplatform) because it
tries to use the native 'gcc' instead of the target compiler.
This fix allows building lgpio for other architectures using
pkgsCross.
[1] https://github.com/joan2937/lg/blob/v0.2.2/Makefile#L4
(cherry picked from commit 2d587a560397d4e33019423249b1bb988d812de3)